Add config validation subcommand

As a user I want to ensure my config is setup correctly. As an APS team dev I want to ensure my user's config set up correctly during troubleshooting or bug reports.

Acceptance Criteria

Add a gwa config validate subcommand. This command will exit the program and print nothing if the application doesn't find any issues or potential warnings. An example output could be:

gwa config validate
[WARN] namespace not set
[ERROR] scheme set to ws, which is invalid. Must only be http or https.

Not required, but output flags of  --json could be added, which would write the output to a JSON file.

Rules

• host: required, must be a valid hostname
• api_key: warning if empty (some operations could be executed without it)
• namespace: warning if empty
• scheme: required, http and https only accepted values
• token_endpoint: required, must be valid URL

Notes

Recommend merging #95 before starting work on this ticket, as that PR introduces a logging pattern that would be useful for this work.