Support Release metadata in pgxn_spec

[theory]

When a distribution is released, PGXN Manager adds three fields to the META.json file for its API, so that clients can validate downloads. Those fields are:

• user: The PGXN username for the user who released the distribution to PGXN.
• date: The timestamp for when the release was made.
• sha1: The SHA-1 hash digest for the release, in hex.

In order for the CLI to be able to validate a download, add support for this data to pgxn/meta as follows:

1. Add a v1 JSON schema that validates these fields.
2. Add a v2 JSON schema that augments the distribution metadata with a new object that contains more/better release metadata, including a URL and flexible SHA representation.
3. Add a struct to support this new field, along with the appropriate accessors.