Add stream subsystem support

[james-callahan]

This is the first plugin to support the stream subsystem, as such some questions have arisen:

• How should you switch between stream and http subsystem operation?
  In this case, I just implemented different :preread vs :rewrite+:access+etc phases
  They purposefully share a log phase
• Should there be a schema field that indicates stream vs http compatibility?

The other question raised by this PR is how to actually propagate tracing contexts in TLS. https://github.com/w3c/trace-context/issues/198 remains unresolved, so this PR currently sets the wire context to nil, I intend to improve on that in a separate PR for the limited use case of kong mesh.

[hishamhm]

How should you switch between stream and http subsystem operation?

At this point, ngx.config.subsystem, which you're using, is the best thing we have. The simplest thing to do would be to expose the same functionality in the PDK, which would be about the best we can do without a larger restructuring of the plugin format.

Should there be a schema field that indicates stream vs http compatibility?

Yes, this is planned but not yet added. So for now we have to make do without it.

[subnetmarco]

@james-callahan I would add some issues to remind ourselves to update the implementation whenever core updates the PDK/schema.