Add target for matrix-bot-sdk

[kegsay]

We have an entire ecosystem relying on https://github.com/turt2live/matrix-bot-sdk for cryptography for bots and appservices. It does NOT use JS SDK, instead relying purely on the rust SDK crypto wrapper https://github.com/matrix-org/matrix-rust-sdk-crypto-nodejs

This wrapper is horribly out-of-date relying on a commit from nearly 1 year ago. As a result, we lack confidence it doesn't have bugs which can cause UTDs. We should add a new b target which will run the bot-sdk, similar to how we have j for js-sdk and r for rust SDK FFI.