Refactor/Reorganize api.js for better tree-shaking

What does this MR do?

At the moment we have one big file for all API calls which doesn't help with tree shaking and for just 4 API calls we include in the main bundle the code for all calls. So this MR reorganizes the api.js to start with by the calls that are used in main.js which should then lead to a 18kb reduction of main.js.

• Duplicates the functions from api.js to specific files per type and specific exports for better tree shaking
• In next step clean up by removing the moved functions from api.js

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