Spike/add type sigs

[ajtucker]

Ruby now has support for writing type signature files and running static type checking. The type signatures can be bundled in a Gem for use in type checking downstream apps.

This PR:

• Adds a dev dependency on steep (the type checker).
• Brings in type signatures for dependent Gems:

  rbs collection init
  rbs collection install

• Generates prototype type signatures from source using rbs prototype:

  for rb in lib/data_services_api/*.rb; do rbs prototype rb $rb > sig/data_services_api/$(basename $rb)s; done

• Sprinkles some type declarations.
• Adds a GitHub Action to run type checks on pull_requests.

For now, we just report any errors, using bundle exec steep check --severity-level=error.

Note that the Value class has been changed slightly: it assumed that hash keys would be symbols, but by default JSON.parse will use strings for keys unless told otherwise. This may affect downstream use, but we should be able to use the type checker to tell us.

The type signatures for dependent gems are not stored in git, but there is a lockfile to ensure that the same versions are downloaded when running the type checker e.g. in the GitHub action. The lockfile can be updated with rbs collection update

[joescottdave]

I'm not really familiar with any of Ruby's type-checking systems, but this is (I'm told) the way the wind is blowing if you want types in Ruby. This gem seems as good a place as any to try them out.

All looks sound to me so far.