feat(otelcol): add support for htpasswd file authentication

[pkarakal]

PR Description

This adds support for using htpasswd for server authentication. Since otelcol.auth.basic is a wrapper around basicauthextension, which already supports both inline and file based server authentication, this adds such support for alloy as well. This would be really useful when we want to have multiple users trying to send data to Alloy and we don't want to a reverse proxy for authenticating calls.

This also makes the username and password fields optional since we should be able to just use htpasswd files for server side authentication. They must be filled in when the handler is used for client authentication.

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.