Support custom WAMP message attibutes

When using the autobahn, how does one set a custom Option when registering an RPC end point?

The RegisterOptions object throws an exception when using an options that is not specified:

https://github.com/crossbario/autobahn-python/blob/fe70ceebe04849470b0c81b26ff6e65e709b0d33/autobahn/wamp/types.py#L930-L935

Section 6.1 of the WAMP specification specifies that if an option is not understood by an implementation then it must be ignored. My understanding is that this means that that the client should transmit the option to the router but not otherwise act on it.

6.1 Extensibility

Some WAMP messages contain Options|dict or Details|dict elements. This allows for future extensibility and implementations that only provide subsets of functionality by ignoring unimplemented attributes. Keys in Options and Details MUST be of type string and MUST match the regular expression [a-z][a-z0-9_]{2,} for WAMP predefined keys. Implementations MAY use implementation-specific keys that MUST match the regular expression [a-z0-9]{3,}. Attributes unknown to an implementation MUST be ignored.

Start at autobahn/wamp/types.py around lines 930-935, where RegisterOptions rejects unspecified options, and read section 6.1 of the linked WAMP specification. Trace registration of an RPC endpoint and verify that an implementation-specific option can be accepted and handled according to the specification without the current exception.