feat: add support for metadata comments in samples

[bcoe]

This adds support for providing additional sample meta-information, based on comments embedded within sample files, like so:

// sample-metadata:
//   title: ACL (Access Control)
//   description: Demonstrates setting access control rules.

Fields we currently intend to support:

• title: a human readable title for the sample.
• description: description of a sample.
• usage: usage instructions for a sample, e.g., node sample.js --hello.
• api_version: what version of the API does this sample apply to, e.g., v1beta, v2.

Future Work

We would like to eventually also support a snippets field, which includes meta information about individual snippets, if multiple snippets are included in the sample file; this has not yet been implemented, as it's not currently used in Node's samples/README.md or README.md, which will be the first two consumers of this functionality.

Note

It's our hope that in the future this meta information will be automatically populated as samples are generated.

CC: @beccasaurus who was also involved in this discussion.

[busunkim96]

Nit: Can we use the 2019 version of the license?