signature: tracking issue for `rand_core` (RandomizedSigner) stabilization

The signature crate contains the RandomizedSigner trait which are presently gated under the rand-preview rand_core feature.

Its main purpose is to allow a CSPRNG to be provided at the time a signature is computed. This is useful with algorithms like ECDSA or RSASSA-PSS which require an RNG at signing time.

There are also lingering concerns that deterministic signature algorithms like Ed25519 or ECDSA when implemented deterministically RFC6979 are brittle in the presence of fault attacks and should supplement their deterministic operation with additional randomness/entropy, which a RandomizedSigner API would allow for.

The main blocker at present is a 1.0 release of the rand_core crate.

Of all of the traits in the signature crate, this one is by far the most underexplored/experimented with. So far there are no crates which actually impl it.

Start by reading the RandomizedSigner trait documentation and reviewing the rand_core 1.0 release dependency described in this issue. The issue is a tracking item rather than a scoped code change; it is complete when the stabilization concerns and dependency blocker have been resolved.