[debug][dap] Support step in command in Python DAP client

Add models and client methods to support step in via the
Python DAP client implementation.

Bug: 532508769
Test: fx test //src/lib/debug/dap/python:client_test
Change-Id: Ic2ee3f0bb0e2d151ff3520d401993f07ee342af7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1735209
Fuchsia-Auto-Submit: Dorian Lee <dorianlee@google.com>
Reviewed-by: Jacob Rutherford <jruthe@google.com>
Commit-Queue: Dorian Lee <dorianlee@google.com>
4 files changed