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

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.