Integrate AWS Code Commit with new syncer

Created by: tsenart

• Integrate AWSCodeCommit with new syncer
  • Implement AWSCodeCommitSource
  • Enable new syncer for AWSCodeCommit external services
  • Use AWS' repositoryId as external_id in repo table for AWS CodeCommit repositories
    • Change ExternalRepoSpec to use repositoryId as ID instead of ARN
    • Write "reset" migration so that syncer writes respositoryId to external_id (see https://github.com/sourcegraph/sourcegraph/pull/3850 for example)
  • Implement awscodecommit.exclude to support enabled state deprecation (this should use AWS repositoryId as id, see previous step)
  • Implement enabled state deprecation migration of AWSCodeCommit external services
    • Migrate all disabled repos to awscodecommit.exclude list.
    • Deprecate awscodecommit.initialRepositoryEnablement setting.

Follow up on #2025 (closed) Part of #3467 (closed)