First release of River Ruby bindings

[brandur]

A first push of Ruby bindings for River, providing insert-only client
to work jobs that are implemented in Go.

Includes two initial drivers in the drivers/ directory, one for
ActiveRecord and one for Sequel, which should cover the vast majority of
Ruby applications making use of Postgres. The drivers are kept in the
main gem's GitHub repository for convenience, but ship as separate gems
so that programs including them can minimize their dependencies.

Overall, I'm happy at how close I was able to keep the API to the Go
version. A lot of syntax in Go just isn't needed due to the more dynamic
and implicit nature of Ruby, but the parts that came through are quite
close. e.g. We have a job args concept, along with InsertOpts that can
be added to both jobs and at insert time, just like Go.

Purposely not implemented on this first push (I'll follow up with these
later on):

• Unique jobs.
• Batch insert.

Try to maintain high Ruby quality standards with:

• Full spec suite that requires 100.0% branch coverage.
• Use standardrb for lint.
• Include RBS files with type and run Steep against the project to
  verify correctness.

[brandur]

Usage instructions in README:
https://github.com/riverqueue/riverqueue-ruby/blob/brandur-first-release/docs/README.md

[brandur]

So actually, I ended up just moving both the ActiveRecord and Sequel drivers into this main repo. This'll keep changes easier to make in the future since everything will be in one place.