[FLI-738] Validate authentication configs at start

[markphelps]

Problem

Authentication configs are not currently fully validated at Flipt startup

For example this config:

authentication:
  methods:
    github:
      enabled: true
      scopes:
        - "user:email"
        - "read:org"
      allowed_organizations:
        - "flipt-io"

Allows Flipt to start up just fine, however, its not a valid config since client_id, client_secret and redirect_address are all required for the OAuth setup with GitHub: https://www.flipt.io/docs/configuration/authentication#github

Ideal Solution

Now that we have per authentication method validation (added https://github.com/flipt-io/flipt/pull/2508/files#diff-9b81b9c76a5ad1bd617096d6af8c6bed965ca6c6b2505ce329d9d74151d0709cR257), we should now define the minimum set of required configuration for each authentication method and then add validation for each auth method we support so that Flipt will not start and warn the user of the invalid/missing fields.

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Additional Context

No response

_FLI-738