feat: bzlmodify

[chickenandpork]

This is a very basic BCR config; there's some remaining work before a BCR submission can be attempted:

1. @1e100 the build seems like it'll likely require google storage credentials to access the archive_gcloud_* repos
2. the tools need to be toolchains and pulled in on demand rather than pre-installed on the build platform. There's a tool that helps that.
3. if you want me to handle the BCR part going forward (it's not like you'll have a huge amount of traffic), add me as a maintainer block in the .bcr/metadata.template.json (see my rules_synology as an example)

Partially fixes #5