websocket negotiation of extension configuration parameters

[lachlan-roberts]

Selecting Extension Parameters

Currently we are overwriting the negotiated ExtenionConfigs with the value of the first offered
ExtensionConfig parameter of the same name extension name. RFC7692 says that the client can request multiple extensions of the same extension name with different configuration parameters to allow the server to choose which set of configuration parameters to use.
https://tools.ietf.org/html/rfc7692#section-5

https://github.com/eclipse/jetty.project/blob/67838609c3059134c3a3374f3de745f66a2ec6d4/jetty-websocket/websocket-core/src/main/java/org/eclipse/jetty/websocket/core/server/Negotiation.java#L234-L247

Here we will overwrite the negotiated extension parameters with the parameters of the first
offered extension. This will make choosing a set of configuration parameters for the extension
impossible.

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Changing Extension Parameters

RFC7629 says that during the negotiation the server may drop or modify extension parameters. In general it looks like whether it is correct to do this is dependant on the specific extension.
https://tools.ietf.org/html/rfc7692#section-5

Server may drop parameters

The contents of the element don't
need to be exactly the same as those of the received extension
negotiation offers.  For example, suppose that a server received a
PMCE extension negotiation offer with an extension parameter "X"
indicating that the client can enable an optional feature named X.
The server may accept the PMCE offer with an element without the
extension parameter "X", meaning that the server chose not to enable
the feature X.  In this case, the offer contains the extension
parameter "X", but the "agreed parameters" don't contain the
extension parameter "X".

Server may change parameters:

The name of the acknowledging parameter
doesn't need to be exactly the same as the offer.  For example, two
parameters, "enable_strong_compression" and
"enable_weak_compression", may be defined as acknowledging parameters
for "enable_compression".