New: Implement MarginFi to Stockpile Trusts

[joeymeere]

The early build of the stockpile-trusts program supports only Kamino, but that's far from the endgame. In vault.rs, you'll see an enum meant to denote a user's protocol of choice, and next up is MarginFi.

The business logic can be handled with simple control flow, however the CPI into their program is what's holding me back. They do not (to my knowledge) have a published crate for v2, and I am too lazy to go through their IDL right now.

If anyone wants to take the plunge on this, I'm happy to help!