Create Context: Helper function to get entities

[iLLiCiTiT]

Changelog Description

Create context has helper functions to collect and cache entities.

Additional info

The need came with ynput/ayon-deadline#49 . That PR is fetching entities on each instance change which slows down the whole logic a lot. This way the entities are cached on CreateContext which avoids unnecessary re-fetching of the same data. The PR needed specifically task type, but I guess there will come soon more that will want to use attributes etc.

The methods are using folder path and task name for filtering as that is what instances have available in data.

The same methods are not used for context validation and to get current context entities.

Testing notes:

1. All should be ok when publisher UI is opened in any host.
2. Validation of context should still happen without any issues.
3. Invalid context on existing instance should be properly displayed as invalid.
4. It is possible to use new helper methods to get corresponding entities.