[HotWallet] Add Fireblocks client

[ian-shim]

This PR adds an http client that makes requests to Fireblocks, supporting two methods for now: list contracts and contract call.
The client will be used to implement a transaction sender that uses remote MPC.

[samlaf]

Added first set of comments. I'm still unsure how this fits with the remote signers framework @shrimalmadhur had created. Is it incompatible, or do you plan on also having this fireblocks client implement the remote signer interface? I'll defer to @shrimalmadhur here as he's the expert who designed and been thinking about remote signing.

[ian-shim]

@shrimalmadhur and I sync'd on the design to support MPC hot wallet.
In summary, the set of API Fireblocks provides doesn't play nicely with the signer interface as it doesn't have a method to sign a transaction without broadcasting it (in a way that is secure). We'll implement a transaction sender interface that will sign + broadcast the transaction.
No matter which interface we end up implementing, this client is necessary to interact with Fireblocks.

[samlaf]

last minor nits

[samlaf]

🚀🚀🚀