Add human friendly fetching endpoints

[jkbpvsc]

Power users have asked to bring back the Bank.getByLabel(string) apis

[losman0s]

This requires bringing back the bank/token metadata fetching to the marginfi-client-v2 package.
They were previously fetched as first-class data in each bank, which was annoying due how decoupled they are from the bank creation process (off-chain vs on-chain).

We should bring it back in a non intrusive way, e.g.:

• add a dedicated but optional metadata field to Bank
• fetch metadata upon instantiation, that can silently fail (although we can issue a warning log if it does, as it should be up)
• bring back the convenience getters which will throw if required metadata is not filled, or succeed and return the right bank