Kestrel requires `process:x_unique_id` but some data sources don't map it

[pcoccoli]

Describe the bug
Kestrel is trying to use x_unique_id (in prefetch) which causes stix-shifter to throw an error

Details of the bug

• What is the hunt flow/script you are executing?

pp = GET process FROM stixshifter://reaqta
     WHERE [ipv4-addr:value = '192.168.1.1']
     START t'2024-03-08T10:45:00Z' STOP t'2024-03-08T11:00:00Z'
     
DISP pp ATTR name

• What is the command that failed?
  The GET command fails.
• What is the error message?

[ERROR] DataSourceError: STIX-shifter translation from STIX to native query failed with message: reaqta connector error => data mapping error : Unable to map the following STIX objects and properties: ['process:x_unique_id'] to data source fields
please check data source config or diagnose with stix-shifter-diag command.

• If it is a STIX-Shifter related issue, what is the results of STIX-Shifter translation?
• If it is a STIX-Shifter related issue, what is the results of STIX-Shifter transmission?

To Reproduce
Steps to reproduce the behavior:

1. setup a data source: e.g. reaqta
2. run hunt flow above

Expected behavior
Kestrel needs some fallback mechanism when an optional extension property like x_unique_id isn't mapped, even if it just ignores the failed prefetch.

Additional context
opencybersecurityalliance/stix-shifter#1650