[feature] Allow configuring Supported in SNI server hook

[epoberezkin]

Currently it is possible to supply server credentials in SNI hook. But Supported should cover all possible credentials. It is probably better to allow overriding or extending Supported in SNI hook?

[dpwiz]

BTW, protocols available in ALPN can also depend on proposed server name. This basically enables full "virtual hosts" for TLS.

[epoberezkin]

@kazu-yamamoto let us know what you think. We are currently working on serving both HTTPS and SMP on the same port (many networks block 5223), and we're already supplying different credentials based on SNI and covering Supported and ALPN, and it probably would be better to completely split server parameters based on SNI.

So if it sounds like reasonable thing for a library to support we could add two more hooks that would return Supported and ALPN, and they will be called in sequence - this would be a backwards compatible change, as servers that don't define the new hooks will continue working as usual.

So in addition to onServerNameIndication, the server would be able to configure onServerNameIndicationSupported and onServerNameIndicationALPN.

Or maybe you disagree it's needed, or have another design idea. Let us know.

[kazu-yamamoto]

As usual, feature requests based on real experience are always welcome.
Would you show your code via a PR?