Consistent Multivariate values for identities

This should be identical across all the SDKs.
Needs to be implemented for:

- [x] Core [Uses Django ID or UUID]
- [x] Edge [Uses Identifier]
- [ ] Python [Uses Composite Key]
- [ ] Rust[Uses composite Key]
- [ ] Golang[Uses Identifier]
- [ ] Java[Uses Composite Key]
- [ ] Node [Django ID or composite Key]
- [ ] Edge proxy[Uses Identifier]
- [ ] Dot Net[Composite Key]
- [ ] PHP[Composite Key]
- [ ] Elixir[Hard to tell, I think composite key]

In order to avoid rolling v2 API's, For core, we will create a flag on environment named `use_mv_v2_evaluation` when turned on will use `composite key` else `identifier if dynamodb_enabled else (id or uuid))`, Edge will use that as flag as well to use `composite_key` if enabled instead of identifier.
Release the Major version for Golang to use composite key.

gz#372