Allow for private key and certificates to be passed into TlsConfig

[Farhie]

TlsConfig requires that your private key and certificate are on the classpath or a file. For instances where we want self signed certs we are generating them during build time, writing them out to a file and then overriding the config passed into the TlsConfig constructor.

It'd be great if we were able to pass in our self signed cert and key.

[jkinkead]

This in particular makes TlsConfig very difficult to use; but it also treats all configuration values as required, without providing fallback defaults. This makes it unsuitable for use as a utility outside of the xio toolchain.

What about switching TlsConfig to a builder, instead of an initialize-once bean class?