Memoize Github::Shell's secret token

## What does this MR do?

`API::Helpers#secret_token` was reading the secret file on every invocation. This MR reads the file in the `gitlab_shell_secret_token.rb` initializer and saves it as a class variable at `Gitlab::Shell.secret_token`

## Are there points in the code the reviewer needs to double check?

 - I'm not sure if the use of `cattr_accessor` is the best approach, or if should be moved into the `class << self` block?
 - Should `API::Helpers#secret_token` be removed in favor of using `Gitlab::Shell.secret_token`?

## Why was this MR needed?

Performance optimization.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22510

See merge request !6599