Use native Gitaly pagination for Branch list API

What does this MR do?

This MR improves the api/project/:id/repository/branches performance by using Gitaly's native pagination gitaly!2251 (merged).

It utilizes page_token(branch name in this case) instead of the default page from Kaminari. It would require API doc changes as well.

Also, as a first iteration, it only works when names or search params were not used.

Before

█ Results summary

* Environment:                Localhost
* Environment Version:        13.2.0-pre `1f57dc1a965`
* Option:                     60s_20rps
* Date:                       2020-07-03
* Run Time:                   1m 1.53s (Start: 07:46:59 UTC, End: 07:48:01 UTC)
* GPT Version:                v2.0.4

NAME                                | RPS  | RPS RESULT       | TTFB AVG  | TTFB P90            | REQ STATUS     | RESULT
------------------------------------|------|------------------|-----------|---------------------|----------------|----------------
api_v4_projects_repository_branches | 20/s | 3.98/s (>3.20/s) | 2472.55ms | 3824.12ms (<7500ms) | 100.00% (>95%) | Passed

After

█ Results summary

* Environment:                Localhost
* Environment Version:        13.2.0-pre `1f57dc1a965`
* Option:                     60s_20rps
* Date:                       2020-07-03
* Run Time:                   2m 2.96s (Start: 07:51:45 UTC, End: 07:53:48 UTC)
* GPT Version:                v2.0.4

NAME                                | RPS  | RPS RESULT       | TTFB AVG  | TTFB P90            | REQ STATUS     | RESULT
------------------------------------|------|------------------|-----------|---------------------|----------------|-----------------
api_v4_projects_repository_branches | 20/s | 2.38/s (>3.20/s) | 4340.72ms | 6978.34ms (<7500ms) | 100.00% (>95%) | FAILED²

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Relates to #208738 (closed)