Slow check-forward-deploy check when there are many tags

[domcleal]

The check-forward-deploy check (and probably check-recent-deploy) is very slow when operating on a repo with over 16,000 tags, taking about 10 minutes.

It tries to determine the last deployment tag for the environment:

fa-harness-tools/lib/fa-harness-tools/check_forward_deploy.rb

Lines 19 to 20 in 2df0cb6

	current_tag = @client.last_deploy_tag(
	prefix: @tag_prefix, environment: @context.environment)

However this relies on loading all tags from the GitHub API with pagination and filtering them by prefix:

fa-harness-tools/lib/fa-harness-tools/github_client.rb

Lines 25 to 28 in 2df0cb6

	@octokit.auto_paginate = true
	@octokit.tags(owner_repo).find_all do |tag|
	tag[:name].start_with?("#{prefix}-#{environment}-")
	end

We need a more optimised way of finding the latest deployment tag. Possibly:

1. Check for a better API, see if we can filter by prefix. If octokit can't do it, check the newer GraphQL API.
2. If we can filter by prefix, we may also need to filter by recent dates as we still have lots of deploy-production prefixed tags.
3. Check for an API that returns a list guaranteed to be sorted.
4. Caching.