Customize where to redirect after GitHub OAuth flow

[bryanhuhta]

When authorizing Grafana Pyroscope with GitHub, we provide a redirect URI which points to where the flow should send us back to after authorizing. At the moment, the URI is hardcoded to be the root url where Grafana is running:

explore-profiles/src/shared/components/GitHubContextProvider/domain/openLoginPopup.ts

Lines 6 to 14 in 0e3a17d

	url.searchParams.set(
	'state',
	btoa(
	JSON.stringify({
	redirect_uri: window.location.origin,
	nonce,
	})
	)
	);

In general, this works. Almost all of our deployments run Grafana at the root url (e.g. tenant.grafana.net). However, this provides a bad experience (possibly even breaking the login flow) when Grafana is not being run at the root of the url (e.g. tenant.grafana.net/grafana). We should make it possible to configure specifically where the redirect will land after the OAuth flow is complete. Ideally, this should be a setting in jsonData of the app plugin.