ci: Add PGXN META.json and release workflow

[theory]

Add a Makefile to automate populating the version in the META.json and creating the zip file. .gitattributes prevents unnecessary files from inclusion in the zip file. .github/workflows/pgxn-release.yml uses the pgxn/pgxn-tools OCI to publish the zip file on PGXN when a server tag is pushed.

To actually publish on PGXN, you'll need to follow these steps:

1. Request a PGXN account. I will approve it ASAP.
2. Once approved, log in and set a password.
3. Navigate to either Actions and secrets for this repo or, if you'd like to use the same credentials to publish other extensions on PGXN, for the ParadeDB org.
4. Hit the "New Repository Secret" or "New Organization Secret" button for each of these two secrets to create them:
   • PGXN_USERNAME: Your PGXN username
   • PGXN_PASSWORD: Your PGXN password

The extension will be published on PGXN the next time you push a tag. If you'd like to publish sooner, simply comment out the tags: ['v[0-9]+.[0-9]+.[0-9]+'] line, push it, let it publish, then uncomment the line to trigger on future tag pushes.

[CLAassistant]

All committers have signed the CLA.

[philippemnoel]

I pushed to the PR to adjust a few of the settings and conform it with your format. I have an account and have configured the secrets. Thank you for pushing this!