MSRV >= 1.76: Tidy up crypto provider customization

In the crypto provider integration code's CryptoProviderBuilder struct (from https://github.com/rustls/rustls-ffi/pull/441), there is an instance of awkward CryptoProvider initialization:

        CryptoProvider {
            cipher_suites,
            kx_groups: self.base.kx_groups.clone(),
            signature_verification_algorithms: self.base.signature_verification_algorithms,
            secure_random: self.base.secure_random,
            key_provider: self.base.key_provider,
        }

We would ideally use the struct update syntax, .. to fill in the non-customized fields (everything but cipher_suites), but we can't do that naively with the Arc<CryptoProvider> we have as a base - we need a CryptoProvider.

Arc::unwrap_or_clone() would allow this in a convenient fashion but we can't use that until our MSRV is >= 1.76, so this issue is a reminder for the future.

Start in the crypto provider integration code at the CryptoProviderBuilder struct and review how its base Arc is used. Once the MSRV is at least 1.76, replace the explicit non-customized field initialization with struct update syntax while preserving the customized cipher_suites field; verify the integration still builds and tests pass.