Automate release process, support NPM publishing

[Siegrift]

Closes #164
Closes #165
Closes #167

Rationale

We agreed on call to publish the packages with the same version (similarly to Airnode). I looked for potential solutions that do this, because such workspace versioning not natively supported by PNPM nor NPM. We can use changesets (what Airnode does) or do this ourselves. I did a small pros/cons:

• Changesets

  • (-) Need to write changesets manually
  • (-) Process to follow
  • (+) Bumps version and does release for projects automatically
  • (+) When done by CI it generates Github release notes per project
  • (+) Generated changelog for each project
  • (+) Tested by many projects in the wild

• Custom Script

  • (-) Code and maintenance overhead
  • (+) Simpler process to follow
  • (+) Single git tag for both projects
  • (+-) Common Github release for both project generated from PR titles

  and ended up with implementing a version script.

  Related

• I've renamed api workspace to signed-api so that the package is the same as on NPM.
• Published packages are @api3/signed-api and @api3/airnode-feed.
• I've tested the publishing locally with Verdaccio. The imports looked good, but I only exported a limited functionality (mostly configuration utils and schemas).