Feature flag for using ring (or aws-lc-rs) instead of openssl in secp256r1-program and its transitive dependents

[ShoyuVanilla]

In some environments, compiling openssl-sys can be problematic.
To address this, I propose introducing a feature flag that allows the use of ring or aws-lc-rs as an alternative to openssl in secp256r1-program and its transitive dependencies.

Although secp256r1-program currently includes a feature flag for vendoring openssl, using ring or aws-lc-rs would provide more flexibility and potentially improve security.

[kevinheavey]

I would prefer p256 because it's more portable (pure Rust) https://crates.io/crates/p256