Provide alternative identification mechanism for automated test cases

[yurish]

Is your feature request related to a problem? Please describe.
Test case identification for automated tests is using summary as ID of the test case. It makes it impossible to change the summary of the testcase after it was generated automatically, or make a summary human-readable if you prepare the case as automated

Describe the solution you'd like
Add reference ID field in the test case, which will be used for test case identification, if it is not empty. Such identification will provide possibility to use any summary you want and will also allow to identify the test case for runner plugins.

Describe alternatives you've considered
Not much alternatives. We can use testcase numerical ID from the database, but I am afraid, that it is not a good solution, because IDs can change if you migrate and numerical ID doesn't bring much information by itself.

Additional context
I am ready to implement this and provide pull request. Just need approve for such changes.

Scope of changes
TestCase model - add new CharField (max _length 255) reference_id
TCMS API - plugin_helpers add reference_id argument to test_case_get_or_create function and use it prior to summary (if reference_id was provided and is not empty)
Start implement this functionality in runner plugins (pytest and other).