sso_proxy: ability to define allowed email address/domain in upstream config

[Jusshersmith]

Problem

Allowed email addresses and domains can only be specified for SSO as a whole, setting the same restriction on all upstreams. It cannot be set on individual upstreams.

Solution

Allow these variables to be defined in two separate places:

• in the environment (in the standard 'options' config), acting as a default setting for all upstreams
• within individual upstream configs, overriding the 'default' setting

We still expect one of these to be set and will error if neither are.

Notes

Includes small bug fix to both the email address validator and email domain validator packages to avoid mutating the passed in slice, instead opting to create and append to a new one.

[codecov]

Codecov Report

Merging #247 into master will increase coverage by 0.08%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   62.28%   62.37%   +0.08%     
==========================================
  Files          50       50              
  Lines        4078     4093      +15     
==========================================
+ Hits         2540     2553      +13     
- Misses       1350     1355       +5     
+ Partials      188      185       -3

Impacted Files	Coverage Δ
internal/proxy/proxy.go	21.95% <0%> (-2.44%)	⬇️
internal/pkg/options/email_address_validator.go	100% <100%> (ø)	⬆️
internal/pkg/options/email_domain_validator.go	100% <100%> (ø)	⬆️
internal/proxy/options.go	84.67% <100%> (+1.07%)	⬆️
internal/proxy/proxy_config.go	78.49% <100%> (+0.23%)	⬆️
internal/proxy/oauthproxy.go	50.48% <0%> (-0.25%)	⬇️