Add DAP server

[rentziass]

This adds a DAP server to the runner to build debugging functionalities. The whole DAP integration is gated by the new EnableDebugger flag on the job message (feature flagged at the API level).

When a job starts, after the job setup we will start the DAP server and allow users to step over to every step in the job, and:

• inspect the scope of the runner
• test evaluating expressions
• run shell commands as if they were run steps in their jobs (full job context, supporting expression expansion, etc.)

Here's an example of what this looks like connecting to the runner from nvim-dap: