on-chain access to currently deployed github commit SHA

[sam0x17]

This is desirable in case we ever lose track of our branches in github or if there is some sort of data loss and we genuinely need to know what is deployed to what.

• [ ] use https://docs.rs/git-version-macro/0.3.9/git_version_macro/macro.git_version.html to write a const storing the current git sha
• [ ] write an RPC OR figure out a way to return this in metadata
• [ ] verify that this can be queried with subxt or whatever the kids use these days
• [ ] OR: some easy way we can make this show up in metadata, would be cool if it just shows up directly in polkadot.js somewhere. As long as the const is in the node or a pallet, should get included in the WASM blob.