WP-API JS Client: Improve support for meta

[adamsilverstein]

The wp.api JavaScript client support for meta needs some attention:

• Meta helper isn't available for comments, terms and users
• Meta pieces haven't been updated for the new-style meta model

The getMeta helper should work correctly for all object types that support meta. We should also consider adding a matching setMeta helper.

See ​https://github.com/WP-API/client-js/issues/168.

[adamsilverstein]

In 41055.diff​ :

Improve support for and detection of meta. wp.api.models.Post, User, Comment and other models supporting meta now include helpers for getMeta() and setMeta().

Some unit tests verifying at a minimum that the helpers are attached would be helpful.

[adamsilverstein]

In 41055.2.diff​:
REST API JS Client - improve support for meta.

• Add getMeta, getMetas, setMeta, setMetas helpers for models that contain meta.
• Add tests for new helpers.
• Include meta data in fixture generation and fixture file driving tests.

[adamsilverstein]

In 41055.4.diff​:

• Add tests: test meta support for Posts, Comments, Tags and Users.
• Add meta for these types to test_build_wp_api_client_fixtures
• Update generated fixture data.

[adamsilverstein]

In 41678:

WP-API JS Client: Improve support for meta.

• Add/fix getMeta, getMetas, setMeta and setMetas helpers for models that support meta.
• Add tests for new helpers, verify meta support for Posts, Comments, Tags and Users.
• Include meta data in fixture generation and fixture file driving tests.

Fixes #41055.