Optionally allow messages without MDC

[aksdb]

Here is my proposal on how to approach #129.

[twiss]

Sorry for the delay! This looks much better.

A few nitpicks: I would name the config option for example InsecureAllowUnauthenticatedMessages, that's a bit more descriptive (for those who don't know what an MDC is), and also makes it clear that the option is insecure. And then the function can just be AllowUnauthenticatedMessages, for example. And the error message could be changed to "Message is not authenticated" (as the "not supported" part is no longer really true after this PR).

[aksdb]

Sorry for the delay! This looks much better.

A few nitpicks: I would name the config option for example InsecureAllowUnauthenticatedMessages, that's a bit more descriptive (for those who don't know what an MDC is), and also makes it clear that the option is insecure. And then the function can just be AllowUnauthenticatedMessages, for example. And the error message could be changed to "Message is not authenticated" (as the "not supported" part is no longer really true after this PR).

Good points. Applied them all.

[twiss]

👍 Thanks!