Draft: Add subcommand functions: serve, proxy, bootstrap

What does this MR do?

Adds a functions subcommand, providing serve, proxy, bootstrap

These sub-commands are ported from step-runner:

• https://gitlab.com/gitlab-org/step-runner/-/blob/main/cmd/bootstrap/bootstrap.go
• https://gitlab.com/gitlab-org/step-runner/-/blob/main/cmd/proxy/proxy.go
• https://gitlab.com/gitlab-org/step-runner/-/blob/main/cmd/serve/serve.go

Why was this MR needed?

Initial support for adding step-runner to Runner.

What's the best way to test this MR?

So far I've only manually tested. We should probably port the tests from step-runner too.

What are the relevant issue numbers?