Summary
This PR introduces optional Authentication of requests to the Metadata Service Layer. By default, this feature is disabled but it can be enabled using the METADATA_SERVICE_AUTH_ENABLED env variable for the datahub-frontend & datahub-gms containers / pods.

This PR description will provide a brief outline of the changes. Read a full summary of these changes here.

Changes

New Components

• Most important: Actor, AuthenticationChain, Authenticator, TokenService, AccessToken
• New Auth Servlet in Metadata Service (/auth) for serving generated access tokens

New Features (UI)

• Now supporting generation of Personal Access Tokens that can be used to make authenticated, programmatic requests to DataHub APIs. Find this under a new 'Settings' tab in the UI.

Refactoring

• Remove usage of RestliEntityClient in GraphQL server, replacement with JavaEntityClient + Entity interface
• Renamed metadata-service:graphql-api module to metadata-service:graphql-servlet-impl to more accurately reflect its contents.
• Renamed metadata-service:restli-impl module to metadata-service:restli-servlet-impl to more accurately reflect its contents.
• Misc. minor naming, annotations changes.

Compatibility
Fully compatible.

Status
Mostly ready for review

Screenshots

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)