feat(rustls-mtls): add valid certificates and tracing subscriber

[jmayclin]

Resolved issues:

Certs are not currently valid and get rejected

ubuntu@ip-172-31-14-132:~/workspace/s2n-quic/examples/rustls-mtls/certs$ openssl x509 -in client.pem -text
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 2 (0x2)
        Signature Algorithm: ecdsa-with-SHA256
        Issuer: CN = quic-test.local CA
        Validity
            Not Before: Jan  9 21:55:45 2023 GMT
            Not After : Mar 10 21:55:45 2023 GMT
        ...

Description of changes:

• adds new certs. These can be regenerated at any times using the generate_certs.sh script.
• adds a tracing subscriber. We currently don't have a vanilla "dump all events to a log file" tracing subscriber example, and it would be nice to have one so that we have an example to point people towards when we ask for them to provide logs.
• adds a Readme

Testing:

Checked the certificate validity dates with openssl, and made sure that the example build and ran.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.