Support loading of locally installed gems

What does this MR do?

Support loading of locally installed gems

Allow administrators to load additional gems during gitlab-rails initialization via gitlab.rb configuration.

This is a general approach that may satisfy many different use cases. Some examples are:

1. Enabling additional OmniAuth strategies. Ref #1420 (closed)
2. Integrations that are specific to on-premise installations and not appropriate for contribution to upstream gitlab-ce.

Note that gems can only be sourced from local directories and must be installed prior to gitlab-ctl reconfigure. This reduces the complexity of the implementation in comparison to one that would install gems from remote sources and ensures the Bundler config can maintain its frozen state between reconfigurations. Administrators can choose which method of installation makes sense for their environment, e.g. Bundler, distro packages, or some configuration management.

If local gems are defined, a Gemfile.local is created that wraps the original Gemfile and defines a path source section for each set of gems in gitlab_rails['gems']. Using path constrains gems to the local path source.

Example configuration:

gitlab_rails['gems'] = {
  '/a/local/dir/of/vendored/gems' => ['some_gem', 'another_gem'],
}

Resulting Gemfile.local:

eval_gemfile("Gemfile")

path "/a/local/dir/of/vendored/gems" do
  gem "some_gem"
  gem "another_gem"
end

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.