The YANG to Protobuf specification mentions using extensions to explicitly set
the field tag of generated Protobuf fields. Add support for annotating field
tags.

* Support setting the field number with occodegenext:field-number.

* Support offsetting the field number with occodegenext:field-number-offset.

* Reject field number annotations that result in a value in the Protobuf
reserved range.

… into support-field-number-gen

Credits to @jasonewang for posting the first implementation at openconfig#339. I've
modified this a bit to read the extension values from the new
openconfig-codegen-extensions.yang model that's now in
openconfig/public, and also to address some of the corner cases that
were discussed in the PR.

Spec:
https://github.com/openconfig/ygot/blob/master/docs/yang-to-protobuf-transformations-spec.md#field-numbering

Corner cases not mentioned in the spec:
1. multiple `field-number` extensions.
  - Error out.
1. multiple `field-number-offset` extensions.
  - Add them up.
1. `field-number` or `field-number-offset` extensions on non-leaves.
  - Silently ignore.
1. `field-number-offset` on non-uses statements, including leafs.
  - Allow.
1. Conflicts.
  - Allow. The proto compilation will of course fail. Automated checking can be added here if we'd prefer.

I made some simple choices for the above items. Erroring out for 3 and 4 would
likely require changing `goyang` to specifically support checking these cases,
which I'm not sure is worth it here.