Replace rand with getrandom and KeysManager entropy in non-test…

[TheBlueMatt]

… code

rand is basically just a way to wrap getrandom with a chacha stream plus a bag of unrelated random utilities. Given we already have a way to wrap getrandom with a chacha stream, there's no point in relying on rand for it, so we simply drop it here.

Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

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[tnull]

Hmm, so I'm not entirely convinced this change is worth the loss of convenience, as it won't allow us to drop rand/rand_core from our dependency tree anyways (as crates like bip39 etc are using it anyways).

So not sure if we gain much by restricting ourselves, and ofc. we risk messing something up?

[TheBlueMatt]

IMO in the long-term we should move towards getrandom over rand, if only to reduce total code size given we already have a RNG expander in the form of the LDK EntropySource. We also want to remove it from rust-secp (rust-bitcoin/rust-secp256k1#881) and IMO should remove it from bip39 as well (there we probably don't need chacha - fetching randomness for root key generation shouldn't need to be stretched using chacha).