deployment: embed build metadata

[travisgroth]

Enable embedding build metadata such as pomerium-cli version or git commit hash. Unfortunately there's no particularly direct way to pass this information into an electron-builder application, so this PR places the information into a new metadata.json file. This file can then be referenced by the desktop UI elements similar to package.json.

The benefit of using a new file over package.json is that we (a) don't have to rewrite an existing file, which feels wrong (b) can avoid accidentally persisting transient changes to package.json

Details:

• Centralize upstream pomerium-cli version to top level package.json
• Add script to generate the new file (./src/metadata.json)
• Add new metadata command to run said script as a dependency for yarn build
• Ensure git ignores /src/metadata.json

It should be easy to add other information such as build time.

This will need a minor rebase onto the in-progress UI refactor if it seems like a sane approach.