chore: Add REGAPIC coverage

[lqiu96]

Thank you for opening a Pull Request! For general contributing guidelines, please refer to contributing guide

Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #1439 ☕️

This is a draft to experiment with having Showcase ITs to cover a bunch of the REGAPIC functionalities.
Showcase modules (currently) intended/ in scope:

• Echo (Unary + Server Side Streaming Calls, Mixins, Timeouts, Retries)
• Compliance (http annotations)
• Identity (CRUD calls)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication

[blakeli0]

I know this PR is still WIP, a general comment is that we don't have to put everything in one PR. As soon as we understand how to write a showcase integration for a certain feature, we can start adding them one by one so it's easier to review and track in the future.

[lqiu96]

Closing as this is being implemented in a series of smaller PRs.