issue267: basic check for removal of schemas

[wtrocki]

Fix for #267

This PR adds a new optional check that allows us to fail when a model is removed.

Testing

./oasdiff -include-checks api-schema-removed -check-breaking -base ./data/openapi-test1.yaml -revision ./data/openapi-test3.yaml

For some reason, localisation is not picked up:

error   [api-schema-removed] at components.schemas.rules
        in API Unknown rules
                messages.api-schema-removed

List of changes

• Add new checker option to readme & optional check flag
• Add the new checker
• Add unit test to the checker
• Update breaking-changes.md file
• Add localization strings and update localizations file with go generate
• Run tests script

Note: please let me know if there is any further tests I can run to validate my PR.

[wtrocki]

To make reviews easier I will leave this PR for maintainers consideration.
Then create separate PR to support differences in refs for request/response body objects that could cover more edge cases.
Then we can have PR that updates this rule to report on endpoints where model refs have been changed.
Rule on its own is quite useful for me right now in current form.

[reuvenharrison]

Fixed localization issue above by running go-localize -input checker/localizations_src -output checker/localizations

[wtrocki]

@reuvenharrison Thank you so much!