Update gnostic to yaml.v3: cleaner parsing, line number reporting

This release updates gnostic to use the yaml.v3 library and the yaml.Node class, which allows us to access line and column numbers of tokens during parsing.

This release upgrades the dependency only and does not include line number reporting.

This change will break clients that directly call auto-generated functions to create OpenAPIv2, OpenAPIv3, and Discovery structures. This is addressed in #195.

Metrics and Protocol Buffer dependency updates

This release provides a tagged version that includes the new metrics directory of protos for API metrics (currently computed using plugins) and updates embedded generated protobuf code to version 3.12.3 of protoc (which hopefully resolves #170).

Improved testing and style compliance

All tests are run by CI. Package directory names are lowercased to match Go style guidance.

OpenAPI 3.1 support

Also streamlined release (removed Swift code for release elsewhere) and added plugin development helpers.

Support for Go Modules

This point release adds go.mod and go.sum files to provide Go module support.

v0.3.0

Bug fixes, enhancements to the OAS v3 surface and removal of gnostic-go-generator (moved to https://github.com/googleapis/gnostic-go-generator)

OpenAPI 3.0.1 support.

This release updates the OpenAPIv3 Protocol Buffer models to OpenAPI 3.0.1 using a JSON schema generated by scraping the text of the official OpenAPI 3.0.1 specification.

The scraper and generated JSON schema are gnostic-specific and are not endorsed by the OpenAPI Technical Steering Committee, which has not yet released or approved a JSON Schema for OpenAPI v3.

Initial release

Tagged for initial use in Kubernetes and Istio.