Make explain accept hash format syntax

[gmcgibbon]

Motivation / Background

This Pull Request has been created because Active Record currently requires adapter specific syntax for explain queries when using the FORMAT option. MySQL uses FORMAT=JSON whereas Postgres uses FORMAT JSON. We should be able to accept a hash that maps to the right format syntax as an option.

Detail

This Pull Request changes explain clause building for mysql and postgres to map hashes to the correct syntax. Example:

Car.all.explain(format: :json)
# or
Car.connection.explain(query, [], [{format: :json}]

Additional information

I previously tried gsub-ing the SQL string but I think this approach with hashes is cleaner.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[gmcgibbon]

I think I actually like the hash syntax better. I'll update the PR to use that instead of mangling SQL.