pw_presubmit: add gn_host_tools step
pw_presubmit: add gn_host_tools step

Add gn_host_tools step. Not included when running
'pw presubmit', but available as 'pw presubmit --step gn_host_tools'.

Also added some type annotations.

Change-Id: I04aeb61b355cff44c4591741ad070e6894519690

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed