[cmd/opampsupervisor] feat: Add extensions framework to supervisor

[dpaasman00]

Description

Adds a framework to the supervisor that allows it to utilize extensions from the collector ecosystem.

• This functionality is blocked behind a feature-gate, opampsupervisor.Extensions, and will need to be enabled to use extensions. The supervisor will exit with error if the extensions config key is present but the feature-gate is not used.
• A new top level key extensions was added to the supervisor configuration.
• Pulled out supervisor configuration validation from the config.Load() method. This better separates responsibility between the Load() and Validate() functions. Previously config.Validate() was being called twice (once when loading the config from file and again when specifically calling Validate() in NewSupervisor()). Doing so also allows us to:
  1. load the supervisor config
  2. check the extensions feature-gate if the extensions key is present in the config
  3. validate the configuration after validating if the feature-gate is valid
• Only one extension is added with this pr, the nop extension, for testing purposes. Follow PRs will add extensions as their use cases are wired into the supervisor (i.e. auth extensions).

Link to tracking issue

Partially closes: #47690

Testing

• Unit tests added
• E2E tests added

Documentation

Supervisor specification updated with usage for extensions.