chore: add branding theme routes

[alejofernandez]

Changes

Please describe both what is changing and why this is important. Include:

Added methods to use the upcoming /branding/themes routes:

GET /branding/themes/default
GET /branding/themes/:theme_id
POST /branding/themes
PATCH /branding/themes/:theme_id
DELETE /branding/themes/:theme_id

References

Please include relevant links supporting this change such as a:

• support ticket
• community post
• StackOverflow post
• support forum thread

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

• This change adds unit test coverage
• This change adds integration test coverage

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[adamjmcgrath]

lgtm, thanks @alejofernandez