[extension/basicauth] Add AWS Secrets Manager support for client credentials

[lazureykis]

Summary

• Add client_auth.aws_secrets_manager option that polls a JSON secret from AWS Secrets Manager and rotates credentials in place without restarting the collector
• Credentials are stored as an atomic pair to guarantee consistency under concurrent reads
• On poll failure, the extension logs a warning and retains the last known good credentials
• Mutually exclusive with existing username/password/*_file options

Resolves #48277

Configuration example

extensions:
  basicauth/client:
    client_auth:
      aws_secrets_manager:
        secret_arn: "arn:aws:secretsmanager:us-east-1:123456789012:secret:my-creds"
        region: "us-east-1"
        refresh_interval: 30m
        username_key: "username"
        password_key: "password"

Test plan

• Unit tests covering initial fetch, atomic pair consistency, rotation detection, error resilience, custom keys, shutdown, and config validation
• All tests pass with -race
• Linter passes

[lazureykis]

Closing this in favor of a different approach suggested by @thomasbaldwin in #48277.

Instead of adding the AWS SDK dependency directly to basicauthextension, I'm first extracting the shared basic auth logic into extension/internal/basicauth (#48414) so that a standalone AWS Secrets Manager auth extension can reuse it without duplicating code. The AWS-specific extension will follow as a separate PR on top of that.