Add docs for omni-transaction-rs + near example

[PiVortex]

Part of #2287

[PiVortex]

This is an example that shows how to use chain signatures with NEAR to enable "Account extensions"

It's a good example that shows how to use omni-transaction-rs inside of a contract. It showcases adding a secp256k1 key to an account, building actions and transactions inside a contract, making a cross-contract call to the MPC, receiving a signature from the MPC, reconstructing the signature and created a signed transaction, sending the signed transaction, and verifying that the transaction was actually successful (not updating state until its received).