feat: add synth rules

[vam-google]

The rules let running synth.py scripts from bazel build. Please check synth/README.md (part of this commit) for more details.

To run synth.py for Language API put the following in the BUILD.bazel file in the //google/cloud/language package:

load("@rules_gapic//synth:gapic_postprocessor.bzl", "java_synth_pkg")

java_synth_pkg(
    name = "java_language",
    synth_script = "synth.py",
    srcs = [
        "//google/cloud/language/v1:google-cloud-language-v1-java",
        "//google/cloud/language/v1beta2:google-cloud-language-v1beta2-java",
    ],
)

The synth.py script ran by the rule above is a regular synth.py script, some parts of which (like templates generation) will result in no-op if they are specific to code publishing and are not about postprocessing/correcting the code generated by the GAPIC generators.