[debug][zxdb-cli] Introduce zxdb-cli schemas

Introduces the `fx debug cli schema` command, which will dump the
zxdb-cli protocol schema.

This is used to generate `schema.json` which now serves as the baseline
for a new golden test. This will be updated as commands are added so
that Agents may be able to investigate the schema directly and
objectively without having to make assumptions from unstructured help text.

Change-Id: Ice4179a6dabf42e683418755769b040a6a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1623795
Fuchsia-Auto-Submit: Jacob Rutherford <jruthe@google.com>
Reviewed-by: Dorian Lee <dorianlee@google.com>
Commit-Queue: Jacob Rutherford <jruthe@google.com>