refactor: enums to use CaseInsensitiveMixin for has_value

[allrob23]

Proposed changes

• Added CaseInsensitiveMixin to improve code reuse for case-insensitive value checking across Enum classes.
• Applied the mixin to the Enuns StixCyberObservableTypes, IdentityTypes, ThreatActorTypes, LocationTypes, ContainerTypes, StixMetaTypes, and MultipleRefRelationship to remove duplicated has_value function logic.
• Updated the logic in has_value to use a set (O(1) lookup) instead of a list (O(N) lookup) and added caching for lower_values to avoid recreating it on every call.

This change follows the DRY principle by centralizing case-insensitive checks in a single mixin, making the code cleaner and easier to maintain. It's not a major performance optimization, but it's a nice improvement. I’ve included a benchmark below to show the results.

Benchmark code and results:
code: https://gist.github.com/allrob23/03382afaffec1a31472ed6fa90fdeb52

Result
Original: 0.004698 seconds
Refactored: 0.000227 seconds

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...