[Fix #227] Make Rails/UniqueValidationWithoutIndex aware of updating schema.rb

[koic]

Fixes #227.

This PR makes Rails/UniqueValidationWithoutIndex aware of updating db/schema.rb

Rails/UniqueValidationWithoutIndex cop needs to know both model and db/schema.rb changes to register an offense. However, with default RuboCop, only changes to the model affect cache behavior.

This PR ensures that changes to db/schema.rb affect the cache by overriding the following method:

# This method should be overridden when a cop's behavior depends
# on state that lives outside of these locations:
#
#   (1) the file under inspection
#   (2) the cop's source code
#   (3) the config (eg a .rubocop.yml file)
#
# For example, some cops may want to look at other parts of
# the codebase being inspected to find violations. A cop may
# use the presence or absence of file `foo.rb` to determine
# whether a certain violation exists in `bar.rb`.
#
# Overriding this method allows the cop to indicate to RuboCop's
# ResultCache system when those external dependencies change,
# ie when the ResultCache should be invalidated.
def external_dependency_checksum
  nil
end

https://github.com/rubocop-hq/rubocop/blob/v0.81.0/lib/rubocop/cop/cop.rb#L222-L239

See for more details: rubocop/rubocop#7496

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• Wrote good commit messages.
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• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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