Add distance to dogs query

[alexander-mathieu]

This PR adds the ability to limit the dogs query by maximum distance in miles. Some changes have been made to the GraphQL folder structure as well.

Changes proposed in this pull request:

• Adds maxDistance argument to dogs query with spec
• Moves GraphQL inputs to their own module/directory, all inputs types are now Types::Inputs::
• Adds an Errors module/directory to the GraphQL directory
• Refactors QueryType to remove dogs query filter logic. Logic has been moved to Helpers::DogsQueryFilterer
• Adds DogsQueryFilterer, which reuses most of the original filter logic, with a few modifications
• Adds default to limit argument for Dogs::sorted_by_distance
• Updates README to include new maxDistance argument for dogs query

Resolves #84

The following checks have been completed:

• [x] Tested my new feature(s) as well as any feasible edge cases (if possible)
• [x] Checked coverage/index.html - did not add any new code that's not covered by testing (if possible)
• [x] Merged in the latest master to my branch with git pull origin master & resolved merge conflicts
• [x] Ran rails db:migrate
• [x] Ran the test suite - all tests are passing (or maybe skipped)
• [x] Checked affected endpoints in Postman / GraphiQL
• [x] Updated README for changes (new endpoints, new gems, etc)