Add get api for a single project_approval_rule

What does this MR do?

Adds missing show api for project_approval_rules

We are in the process of adding approval rules to the GitLab Terraform provider

Right now, we are having to query the index path and loop through all approval rules to find a single rule. Having the ability to query a single approval rule would improve the process.

This MR will:

• Add a GET path to the API where a user can query a single approval rule
• Update associated tests and docs

Screenshots (strongly suggested)

approval

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