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).
⚠️ Report is 5 commits behind head on master.

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 

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[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

[stevehipwell]

My changes have been implemented and I'm happy to approve but I think @alexandear did a more thorough review so his approval would likely carry more weight.

[gmlewis]

Thank you, @stevehipwell and @alexandear!
Merging.