feat: Add support for Enterprise GitHub App Installation APIs

[Not-Dhananjay-Mishra]

Relates-to: #3829

This PR add support for the following API endpoints -

• GET /enterprises/{enterprise}/apps/installable_organizations
• GET /enterprises/{enterprise}/apps/installable_organizations/{org}/accessible_repositories
• GET /enterprises/{enterprise}/apps/organizations/{org}/installations
• POST /enterprises/{enterprise}/apps/organizations/{org}/installations
• DELETE /enterprises/{enterprise}/apps/organizations/{org}/installations/{installation_id}

REST API endpoints for Github App Installation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.41%. Comparing base (148abf3) to head (884c2d1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3830      +/-   ##
==========================================
+ Coverage   92.38%   92.41%   +0.03%     
==========================================
  Files         196      197       +1     
  Lines       14094    14152      +58     
==========================================
+ Hits        13021    13079      +58     
  Misses        884      884              
  Partials      189      189              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @Not-Dhananjay-Mishra!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @zyfy29

[stevehipwell]

I've just reviewed the API surface and I think some of the function names need re-working before doing a deeper review.

[Not-Dhananjay-Mishra]

@stevehipwell Thanks for the suggestions, I have change the name for both -
ListInstallableEnterpriseOrganization -> ListInstallableAppOrganizations
ListOrganizationAccessibleRepositories -> ListOrganizationAppAccessibleRepositories

[stevehipwell]

I still don't think the API is consistent, how about the following changes to make the API consistent by using an App prefix with all unnecessary padding removed?

• ListInstallableAppOrganizations -> ListAppInstallableOrganizations
• ListOrganizationAppAccessibleRepositories -> ListAppAccessibleOrganizationRepositories
• ListAppOrganizationInstallations -> ListAppInstallations
• InstallEnterpriseOrganizationApp -> InstallApp
• UninstallEnterpriseOrganizationApp -> UninstallApp

[Not-Dhananjay-Mishra]

* `ListInstallableAppOrganizations` -> `ListAppInstallableOrganizations`

* `ListOrganizationAppAccessibleRepositories` -> `ListAppAccessibleOrganizationRepositories`

* `ListAppOrganizationInstallations` -> `ListAppInstallations`

* `InstallEnterpriseOrganizationApp` -> `InstallApp`

* `UninstallEnterpriseOrganizationApp` -> `UninstallApp`

I have implemented the following changes as requested

[Not-Dhananjay-Mishra]

@alexandear Thanks for all suggestions, I made some changes as requested -

• added missing field in InstallableOrganization
• AppInstallationRequest -> InstallAppRequest
• Repository -> Repositories in InstallAppRequest
• fix *string and omitempty usage in both struct