support clear text cred configuration

[ImpostorKeanu]

Note

This should merge cleanly with #191

1. What does this change do, exactly?

This change allows non-Caddyfile configurations to supply basic credentials in clear text.

• The AuthCredentials field is currently set to [][]byte, which must be base64 encoded by the JSON package.
• When configuring via JSON, this forces credentials to be b64 encoded in the configuration file.
• UnmarshalCaddyFile sidesteps this by encoding these values manually.

Implementing a slice of strings at configuration enables support for consistent input values.

2. Please link to the relevant issues.

N/A

3. Which documentation changes (if any) need to be made because of this PR?

N/A

4. Checklist

• I have written tests and verified that they fail without my change
• I made pull request as minimal and simple as possible. If change is not small or additional dependencies are required, I opened an issue to propose and discuss the design first
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code