Add GitHub API cache to avoid rate limit

This will cache any GitHub API responses with correct Cache-Control header.

`gregjones/httpcache` has been chosen as a library to implement this feature, as it is as recommended in `go-github`'s documentation:

https://github.com/google/go-github#conditional-requests

`gregjones/httpcache` supports a number of cache backends like `diskcache`, `s3cache`, and so on:

https://github.com/gregjones/httpcache#cache-backends

We stick to the built-in in-memory cache as a starter. Probably this will never becomes an issue as long as various HTTP responses for all the GitHub API calls that ARC makes, list-runners, list-workflow-jobs, list-runner-groups, etc., doesn't overflow the in-memory cache.

`httpcache` has an known unfixed issue that it doesn't update cache on chunked responses. But we assume that the APIs that we call doesn't use chunked responses. See #1503 for more information on that.

Ref #920

github/github.go

@@ -13,6 +13,7 @@ import (
 	"github.com/actions-runner-controller/actions-runner-controller/github/metrics"
 	"github.com/bradleyfalzon/ghinstallation"
 	"github.com/google/go-github/v39/github"
+	"github.com/gregjones/httpcache"
 	"golang.org/x/oauth2"
 )
 
@@ -82,8 +83,10 @@ func (c *Config) NewClient() (*Client, error) {
 		transport = tr
 	}
 
-	transport = metrics.Transport{Transport: transport}
-	httpClient := &http.Client{Transport: transport}
+	cached := httpcache.NewTransport(httpcache.NewMemoryCache())
+	cached.Transport = transport
+	metricsTransport := metrics.Transport{Transport: cached}
+	httpClient := &http.Client{Transport: metricsTransport}
 
 	var client *github.Client
 	var githubBaseURL string