bp2build: add bazel_module: { bp2build_available } prop.

This CL adds a per-target allowlist to instruct bp2build on which modules it should generate Bazel targets for.

Test: soong tests
Change-Id: I869e66fce405c2c6689b381569b8cc0118cbcf76
diff --git a/bazel/properties.go b/bazel/properties.go
index fbbbfe6..65f37e3 100644
--- a/bazel/properties.go
+++ b/bazel/properties.go
@@ -17,6 +17,9 @@
 type bazelModuleProperties struct {
 	// The label of the Bazel target replacing this Soong module.
 	Label string
+
+	// If true, bp2build will generate the converted Bazel target for this module.
+	Bp2build_available bool
 }
 
 // Properties contains common module properties for Bazel migration purposes.