feature: Added better support for testing with AJV having discriminator option turned on

[heath-freenome]

Reasons for making this change

A recent issue made it clear that we didn't make it easy for users to turn on discriminator support from AJV

• In @rjsf/utils improved support for discriminator as follows:
  • Updated the ValidatorType to add support for an optional reset() method
  • Updated the ParserValidator to implement reset() to clear the schema map, including a test to verify that
  • Updated the tests that used discriminator to remove the mapping block that AJV doesn't support
  • Updated the getFirstMatchingOption() test to deal with the situation where AJV doesn't support discriminator for array types
  • Updated the retrieveSchema() test to call reset() on validators that have it in an afterEach()
  • Updated the getTestValidator() implementation to implement a reset() that empties the arrays
• In @rjsf/validator-ajv8 improved support for discriminator as follows:
  • Updated the createAjvInstance() function to denote that we want to make discriminator: true the default in v6
  • Updated the AJV8Validator to make reset do ajv.removeSchema() to clear the cached schemas
  • Updated the getTestValidator() implementation to call reset()` on the validator if it exists
  • Updated the schema.test.ts file to run a set of test with discriminator: true set on the AJV8Validator
• Updated the CHANGELOG.md file accordingly

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests. I've run npx nx run-many --target=build --exclude=@rjsf/docs && npm run test:update to update snapshots, if needed.
  • I've updated docs if needed
  • I've updated the changelog with a description of the PR
• I'm adding a new feature
  • I've updated the playground with an example use of the feature