bp2build: add configurable attribute (select) support.

This CL adds a basic framework to support configurable string_list
attributes, selecting on the Arch variant (x86, x86_64, arm, arm64).

It offers fine-grained controls to map individual configurable
properties (arch_variant) to configurable Bazel attributes, starting
with the string_list type for the copts property for cc_object.

This design is primarily motivated to have minimal boilerplate in
bp2build mutators, allowing anyone to opt-in configurable attributes,
and modify intermediate states before passing them on into the
CreateBazelTargetModule instantiator.

Fixes: 178130668

Test: go tests
Test: build/bazel/scripts/milestone-2/demo.sh

Change-Id: Id6f04d7c560312a93e193d7ca4e1b7ceb6062260