Response caching

[reny-gearset]

Can we provide a way to cache the response by passing in https://github.com/nigel-sampson/octokit.caching/blob/master/Octokit.Caching/ICache.cs into the GithubClient constructor?

The caching interface can be more specific as follows:

• for getting the cached response, we can have something like public async Task<IResponse?> GetAsync(IRequest request)
• for setting the cached response, we can have something like public async Task SetAsync(IRequest request, IResponse response)

Project reference:
https://github.com/nigel-sampson/octokit.caching
https://github.com/nigel-sampson/octokit.caching/blob/master/Octokit.Caching/CachingHttpClient.cs

[reny-gearset]

Initial attempt to implement the caching layer: #2648