KIC review and document plugin protocols behavior

[rainest]

Originally raised internally in FTI-2517.

Kong plugins support a protocols field indicating which request protocols (e.g. http, grpc) they run on. KongPlugin resources have supported this field for a while (since 0.4), but we do not mention it in documentation.

The Kong and KIC defaults also diverged at some point. Kong currently defaults to running on http,https,grpc,grpcs, but KIC defaults to http,https. I didn't immediately find where this default is coming from. Initial review of the KIC code indicates that it should just use whatever value is in the KongPlugin (or nothing if empty, allowing Kong to populate its default). decK does not have a standard constant default for it.

We should review the plugin handling code to determine where this value is currently being set, update it to reflect the current Kong default, and add the field to documentation.