Allow adding `@` prefix for internal projects

[minwoox]

Motivation: The xDS module requires its own Central Dogma project for managing xDS resources. While it's possible to create a project with an arbitrary name that is not currently is use, adding a @ prefix to the project name has several benefits:

• It allows adding other internal projects later without worrying about duplicate names
• It makes it easy to distinguish internal projects from non-internals.

Modifications:

• Split valid name pattern to USER_INPUT_PROJECT_AND_REPO_NAME_PATTERN and PROJECT_AND_REPO_NAME_PATTERN
  • USER_INPUT_PROJECT_AND_REPO_NAME_PATTERN is used to validate the names entered by users.
  • PROJECT_AND_REPO_NAME_PATTERN is used internally.

Result:

• You can now create an internal project whose name starts with @.

[minwoox]

This version has to be deployed before #965 is deployed.

[ikhoon]

Still LGTM