feat: Add Identity Insights

[superchilled]

This PR implements the Identity Insights API in the Ruby SDK. Specifically it:

• Defines an IdentityInsights class with a requests instance method to make Identity Insights requests
• Defines an IdentityInsights::InsightsBuilder class with methods to add required insights to the request
• Update the Client class to add an identity insights instance method which returns an IdentityInsights object
• Adds unit tests to cover the new functionality
• Does some tidy up of existing test files not related to Identity Insights

[alnacle]

LGTM 🚀