Extract projects related logic from Sentry

What does this MR do?

The Sentry::Client starts getting too many responsibilities. This this MR I would like to start a series of refactoring to split a Sentry::Client into separate classes/modules.

This MR extracts projects related logic into a separate module.

This MR does not refactor the initial implementation of that functionality (except renaming a list_projects method to projects). It was not refactored for a reason. I would prefer to do it separately to keep the current MR smaller.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team