add valid_until to Client

[goenning]

fixes #1675

I found 3 different ways of doing this, but I'm not sure which one is the best.

This PR implements Option 1, but I'm fine with other options too (except maybe 3, my least favorite)

• Option 1: Fetch the identity_pem during the make_generic_builder and pass that onto both rustls and openssl HttpsConnector functions as an arg. This is a breaking change for those who are NOT using the make_generic_builder function.
• Option 2: Keep the exec_identity_pem call in both rustls and openssl connector builder as an arg, and bubble up the expiration datetime to the make_generic_builder function. This is also a breaking change for those who are using the make_generic_builder function as the return type for the HttpsConnector functions will change.
• Option 3: Call exec_identity_pem twice, once in the make_generic_builder function just to get the expiration date, and again inside the rustls and openssl HttpsConnector functions (as it is currently). This is NOT a breaking change, but it is inefficient as we are calling the exec_identity_pem function twice as this is a blocking system call.

[goenning]

will work on fixing tests soon, just wanted to get your thoughts on the options