Add TLS 1.2 support via configuration

[plv]

Description

N.B.: This PR is combined is meant to be reviewed alongside https://github.com/WalletConnect/a2/pull/91

This PR adds a TLS 1.2 override to ClientConfig. This is useful in cases where we want to use a signing scheme that is not supported by TLS 1.3.

Resolves # (issue) N/A

How Has This Been Tested?

I've pulled my fork into our own internal codebase and tested this by signing using RSA PKCS1v15 signing scheme, something that is NOT supported by TLS 1.3 for client auth. Got push notifications successfully sending to my device!

* Writing a unit test is fairly difficult here, so please let me know if you'd like me to add to examples/certificate_client.rs

Due Dilligence

• [ ] Breaking change
• [ ] Requires a documentation update
• [ ] Requires a e2e/integration test update

[plv]

@chris13524 Could I get a review on this and #91? Thank you :)