AE 1236: add support for Object metric type in Go

[symhom]

We might use the Object type in Go, but it is currently not supported.

Things to point out

• See my comment about exit versus exit(). I'm not sure if that's right.
• oneOf is not supported
• It uses inline nested structs for nested objects. So in the below example, Metadata is an anonymous function that is nested, as opposed to being its own struct. This is to prevent name collision.

type TestNestedObjectObject struct {
	UserId string `json:"user_id"`
	Metadata struct {
		Version float64 `json:"version"`
		Active bool `json:"active"`
	} `json:"metadata"`
	Tags []string `json:"tags"`
}

Pull Request checklist

• Quality: This PR builds and tests run cleanly
  • make test runs without emitting any warnings
  • make lint runs without emitting any errors
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
  • Any breaking changes to language binding APIs are noted explicitly