Move TLS authentication material to registry credentials object

[wagoodman]

#169 added support for providing client certificate and key for MTLS authentication with a registry as well as the ability to validate cert chains with a self-signed cert. This augments that PR by moving this client cert configuration to a per-registry configuration while expanding the ca cert configuration on registry options.

Though the CAFile on the go tlsconfig options does not replace the root cert pool with the single user-provided cert, having a singular field for this (instead of per-registry) prevents the ability to provide multiple CA certs for use with multiple registries that may have certs from unrelated CAs. The larger issue is that ClientCert would be presented to any registry, even though it should only apply to a single registry.

The specific changes made are:

• change the CAFile option to allow for either a single cert file or directory of *.crt, *.cert, *.pem files (new item is CAFileOrDir).
• Move ClientCert and ClientKey to the RegistryCredentials object.
• Add RegistryOptions.TLSOptions to handle crafting TLS options for MTLS.
• Add ability to select and order the most specific credentials based on authority. This means that config items with a matching authority are preferred over config items with no authority specified.

[github-actions]

Benchmark Test Results

Benchmark results from the latest changes vs base branch

latest: Pulling from library/ubuntu
b237fe92c417: Pull complete
tar: Option --mtime: Treating date 'UTC 2019-09-16' as 2019-09-16 00:00:00
goos: linux
goarch: amd64
pkg: github.com/anchore/stereoscope/pkg/file
cpu: Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz
docker: 
           │ ./.tmp/benchmark-a402d50.txt │
           │            sec/op            │
TarIndex-2                   53.21µ ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-a402d50.txt │
           │             B/op             │
TarIndex-2                  5.561Ki ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

           │ ./.tmp/benchmark-a402d50.txt │
           │          allocs/op           │
TarIndex-2                    93.00 ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

pkg: github.com/anchore/stereoscope/test/integration
                                      │ ./.tmp/benchmark-a402d50.txt │
                                      │            sec/op            │
SimpleImage_GetImage/docker-archive-2                   1.915m ± ∞ ¹
SimpleImage_GetImage/oci-archive-2                      1.784m ± ∞ ¹
SimpleImage_GetImage/oci-dir-2                          1.300m ± ∞ ¹
geomean                                                 1.643m
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-a402d50.txt │
                                      │             B/op             │
SimpleImage_GetImage/docker-archive-2                  325.2Ki ± ∞ ¹
SimpleImage_GetImage/oci-archive-2                     645.9Ki ± ∞ ¹
SimpleImage_GetImage/oci-dir-2                         413.1Ki ± ∞ ¹
geomean                                                442.7Ki
¹ need >= 6 samples for confidence interval at level 0.95

                                      │ ./.tmp/benchmark-a402d50.txt │
                                      │          allocs/op           │
SimpleImage_GetImage/docker-archive-2                   2.658k ± ∞ ¹
SimpleImage_GetImage/oci-archive-2                      1.570k ± ∞ ¹
SimpleImage_GetImage/oci-dir-2                          1.354k ± ∞ ¹
geomean                                                 1.781k
¹ need >= 6 samples for confidence interval at level 0.95

docker: Error response from daemon: Get "http://localhost/v2/": dial tcp [::1]:80: connect: connection refused.
                                                   │ ./.tmp/benchmark-a402d50.txt │
                                                   │            sec/op            │
SimpleImage_FetchSquashedContents/docker-archive-2                   21.77µ ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-a402d50.txt │
                                                   │             B/op             │
SimpleImage_FetchSquashedContents/docker-archive-2                  2.648Ki ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95

                                                   │ ./.tmp/benchmark-a402d50.txt │
                                                   │          allocs/op           │
SimpleImage_FetchSquashedContents/docker-archive-2                    21.00 ± ∞ ¹
¹ need >= 6 samples for confidence interval at level 0.95