add auth-by-hmac plugin

[axos88]

Signed-off-by: Akos Vandra-Meyer [email protected]

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Added a medium-complexity auth plugin that allows authenticating users without the broker having prior knowledge of the actual usernames. Can be usefule in constrained environments where certificate based authentication is not an option. The plugin source code describes the uses and caveats in detail.

Can also serve as an example plugin.

Not sure about a CMakeLists file, never used cmake, so I just copy pasted and adjusted the auth-by-ip plugin.

[axos88]

Just signed the ECA again

[axos88]

Not sure what is wrong here.