Implement methods for .sql validation source

What does this MR do and why?

Previously you could only run the schema validation checker diff against a structure.sql and an actual database, but it is useful to run a diff between two different SQL files. Previously this did not work because the methods implemented by the database source (such as fetch_index_by_name) did not exist.

This commit adds those methods and refactors the existing methods to use them.

How to set up and validate locally

On macOS, I needed !195581 (merged) to build a more recent version of pg_query.

Attached are two .sql files:

17.9.3.sql

mysql-17.9.2.sql

I was able to generate a diff via:

require 'gitlab/schema/validation'

good_sql = Gitlab::Schema::Validation::Sources::StructureSql.new('17.9.3.sql')
bad_sql = Gitlab::Schema::Validation::Sources::StructureSql.new('mysql-17.9.2.sql')
validators = Gitlab::Schema::Validation::Validators::Base.all_validators

inconsistencies =
  Gitlab::Schema::Validation::Runner.new(good_sql, bad_sql, validators: validators).execute

inconsistencies.each do |inconsistency|
  puts inconsistency.display
end

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