gerritstatusupdater: add logic for handling Dispatch-Trailer

If we receive an event for a commit that contains a Dispatch-Trailer,
then decode that trailer in preference to trying to parse information
from the branch name for the event.

There is a lot going on in this CL. Ideally it would have been split
into separate CLs for easier review etc. However, we have no
unit/integration tests for gerritstatusupdater. The only way we can test
the thing is to hook it up locally using ngrok and fire some events
through it with real CI builds on GitHub. Splitting up this CL into
multiple CLs would have required doing that manual testing many times
over. So hopefully the assurance that this has been tested heavily
locally, in all three modes of GERRITSTATUSUPDATER_DISPATCH_TRAILER_MODE
is sufficient assurance.

Signed-off-by: Paul Jolly <paul@myitcv.io>
Change-Id: I3a3a5a9b4e4efd3745e67b9ebd32badb950d95f5

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/andygrunwald/go-gerrit v0.0.0-20220917070846-2e881e2fb2b5
 	github.com/apex/gateway v1.1.2
 	github.com/aws/aws-lambda-go v1.32.1
+	github.com/google/go-cmp v0.5.8
 	github.com/google/go-github/v45 v45.2.0
 	github.com/rogpeppe/testscript v1.1.0
 	golang.org/x/mod v0.6.0-dev.0.20220818022119-ed83ed61efb9

go.sum

@@ -21,6 +21,7 @@ github.com/golang/protobuf v1.3.2 h1:6nsPYzhq5kReh6QImI3k5qWzO4PEbvbIW2cwSfR/6xs
 github.com/golang/protobuf v1.3.2/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 github.com/google/go-cmp v0.5.2/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-cmp v0.5.8 h1:e6P7q2lk1O+qJJb4BtCQXlK8vWEO8V1ZeuEdJNOqZyg=
+github.com/google/go-cmp v0.5.8/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/google/go-github/v45 v45.2.0 h1:5oRLszbrkvxDDqBCNj2hjDZMKmvexaZ1xw/FCD+K3FI=
 github.com/google/go-github/v45 v45.2.0/go.mod h1:FObaZJEDSTa/WGCzZ2Z3eoCDXWJKMenWWTrd8jrta28=
 github.com/google/go-querystring v1.1.0 h1:AnCroh3fv4ZBgVIf1Iwtovgjaw/GiKJo8M8yD/fhyJ8=