refactor: remove RandomCoin

[juan518munoz]

We are currently investigating whether it's possible to remove custom randomness from crypto:

Why do we have our own Rng traits? Can't we implement Rng from rand on types directly?

0xMiden/crypto#1025 (comment)

Part of this effort is to see if the client can get away without it. In this PR two things are important in that respect:

1. We can replace RandomCoin with a client's own struct: DefaultFeltRng (ChaCha20).
2. Yet we still depend on the FeltRng trait itself, as some signatures need to satisfy it (note creation and TransactionRequestBuilder take impl FeltRng). For this we'd need to replace the places where FeltRng is required with something more generic like Rng, which implies making changes to both protocol and node*.

• biggest setback about this is the need of switching back node's protocol dependency to next, which would bring breaking changes to devnet (thus postponed momentarily).

Tackles #2204