feat: add device identifiers to rokt placements

[nickolas-dimitrakas]

Background

• Rokt requires device identifiers (IDFA) for cross-app attribution tracking in ad placements
• Previously, only user attributes were passed to Rokt placements, missing device-level identifiers

What Has Changed

• Added IDFA and IDFV to Rokt placement attributes: Automatically includes IDFA and IDFV from user identities when calling selectPlacements
• Replaced magic strings with constants: Created kMPRoktAttributeKeyIDFA, kMPRoktAttributeKeyIDFV, kMPRoktAttributeKeySandbox, and internal config key constants for maintainability
• Added comprehensive method documentation: Documented all public and private methods in MPRokt.m
• Updated tests: Modified existing tests (testSelectPlacementsSimpleWithValidParameters, testSelectPlacementsExpandedWithValidParameters, testSelectPlacementsSimpleWithMapping) to verify IDFA and IDFV is correctly added when available

Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have tested this locally.

Reference Issue (For employees only. Ignore if you are an outside contributor)

• Closes NO JIRA

[jamesnrokt]

Really great refactor with a few minor comments

[thomson-t]

There is currently a bug in the rokt-kit that will add the attributes only from the user object. I think @BrandonStalnaker is going to address it soon.

[BrandonStalnaker]

This functionality may already exist in the sdk, though it's possible its not using the key Sam wants or that it has a bug. I'm going to test through and confirm the functionality asap.
mparticle-integrations/mparticle-apple-integration-rokt#8