Add debug logging to help with troubleshooting

[stmcginnis]

Issue #, if available:

Related: #216

Description of changes:

This adds a --verbose flag to the CLI to enable debug level log output. This can be useful when something is failing or taking a long time. With normal execution there is no indication of what is happening. This will at least give some sort of troubleshooting step a user can take to help triage what is happening.

Testing

Performed all commands with and without the --verbose flag and verified proper operation and output.

Also verified the use of the RUST_LOG environment variable to enable other tracing that could be useful for developers troubleshooting issues:

$ RUST_LOG=aws_config=debug cargo run --bin coldsnap -- upload /tmp/1000604764 
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/coldsnap upload /tmp/1000604764`
[INFO ] load_region; provider=EnvironmentVariableRegionProvider { env: Env(Real) }
[INFO ] load_region; provider=ProfileFileRegionProvider { fs: Fs(Real), env: Env(Real), profile_override: None }
[DEBUG] loaded home directory src="HOME"
[DEBUG] performing home directory substitution home="/home/stmcg" path="~/.aws/config"
[DEBUG] home directory expanded before="~/.aws/config" after="/home/stmcg/.aws/config"
[DEBUG] config file loaded path=~/.aws/config size=29
...

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[stmcginnis]

Package version conflicts in dependencies.

I see we already have tracing as a dependency of a dependency, so I'll take a look at using that directly rather than bringing in the logging packages.

[ecpullen]

LGTM