Add tests and examples for mutual TLS

[Scottmitch]

Motivation:
We currently do not have unit tests and examples dedicated to mutual TLS.

Modifications:

• Add client/server example for HTTP which demonstrates mutual TLS
• Add HTTP unit test which verifies expected behavior for mutual tls
• Update scripts to generate client/server variants of certificates and
  generate the new certificates

Result:
Unit test and examples exist for mutual TLS

[idelpivnitskiy]

Good that you added separate certs for the client. Can you please also use new client certs here?

servicetalk/servicetalk-tcp-netty-internal/src/test/java/io/servicetalk/tcp/netty/internal/SecureTcpTransportObserverErrorsTest.java

Line 115 in 0a60ec9

clientSecurityConfig.keyManager(DefaultTestCerts::loadServerPem, DefaultTestCerts::loadServerKey);