[Go] factor out Gemini plugin common code (#612)

The two Gemini plugins share a lot of code, but we can't write
a common package because the genai packages they import are different.
Instead, we copy code from one package to the other.

The copy command in this PR is a simple tool that copies sections
of one file to another.

We mark up the googleai and vertexai files to use this tool, treating
the googleai files as the source of truth.

There is more code to factor out, and we will do that in later
PRs. This is just a start.

go/go.mod

@@ -24,6 +24,7 @@ require (
 	go.opentelemetry.io/otel/sdk/metric v1.26.0
 	go.opentelemetry.io/otel/trace v1.26.0
 	golang.org/x/exp v0.0.0-20240318143956-a85f2c67cd81
+	golang.org/x/tools v0.23.0
 	google.golang.org/api v0.188.0
 	google.golang.org/protobuf v1.34.2
 	gopkg.in/yaml.v2 v2.4.0

go/go.sum

@@ -223,6 +223,10 @@ golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGm
 golang.org/x/tools v0.0.0-20190226205152-f727befe758c/go.mod h1:9Yl7xja0Znq3iFh3HoIrodX9oNMXvdceNzlUR8zjMvY=
 golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
 golang.org/x/tools v0.0.0-20190524140312-2c0ae7006135/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
+golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d h1:vU5i/LfpvrRCpgM/VPfJLg5KjxD3E+hfT1SH+d9zLwg=
+golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d/go.mod h1:aiJjzUbINMkxbQROHiO6hDPo2LHcIPhhQsa9DLh0yGk=
+golang.org/x/tools v0.23.0 h1:SGsXPZ+2l4JsgaCKkx+FQ9YZ5XEtA1GZYuoDjenLjvg=
+golang.org/x/tools v0.23.0/go.mod h1:pnu6ufv6vQkll6szChhK3C3L/ruaIv5eBeztNG8wtsI=
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 google.golang.org/api v0.188.0 h1:51y8fJ/b1AaaBRJr4yWm96fPcuxSo0JcegXE3DaHQHw=
 google.golang.org/api v0.188.0/go.mod h1:VR0d+2SIiWOYG3r/jdm7adPW9hI2aRv9ETOSCQ9Beag=