cli: add more execution output format options

[bduffany]

Support more flexible output options in bb execute, and add a bb executions get command for fetching execution details for an execution ID, supporting the same new output options as bb execute. This makes it easier to look at server logs etc. from ad-hoc bb execute commands, or from execution IDs that we see in logs (especially useful for workflow execution debugging since failed workflows don't always display properly in the UI for various reasons).

(re. bb executions, I went with a subcommand approach because at some point it'll probably be useful to add bb executions wait, bb executions status, and bb executions cancel, but didn't think it was worth bloating bb --help with all of these different commands)

• Add a new flag --output={id|stdio|json|md|markdown} which defaults to stdio (stdout + stderr) for bb execute, which is what we show today, and defaults to markdown for bb executions get.
• id output prints just the execution ID and skips fetching any additional data from cache. This is useful for testing lots of executions and then later analyzing them using a separate pipeline (e.g. in parallel)
• The json output contains more detailed execution info - similar information to we show in the execution card in the UI, including stdout, stderr, server logs, timing information, and metadata. This is useful for scripting.
• The markdown/md format contains a human-readable presentation of the JSON format (i.e. it's optimized for readability). Example:

[maggie-lou]

Non-blocking and there's a general CLI documentation hole, but it would be nice to have internal docs about using this tool to debug

Generally LGTM - some small comments

[maggie-lou]

Maybe these should go in mardown parsing util?

[bduffany]

Sounds good - I added a cli/markdown package that gives us a markdown.Writer - it accepts a stream of markdown and automatically colorizes it. I figured it's easier to just write markdown to a writer and not have to worry about colors / formatting etc.

Right now it just colorizes headers - later we can get fancier if we want. If we wind up needing this for other stuff too, we can move it to server/util.