WIP: Backend for Sentry Project selection dropdown

What does this MR do?

1. Add Controller endpoint to get Sentry project list, given Sentry host and token.
2. Make call to Sentry API to get Sentry project list.
3. Modify the settings controller endpoint to receive Sentry host, project name & organization name as separate values and save into DB.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/55199

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #55199 (closed)