Additions to DataSource model

[ljstella]

splunk/security_content#3287 <- PR to add fields to the datasource object.

At the moment, I've typed these incredibly naively with separator_value as an Optional[str], and mitre_components as list[str]. I think separator_value may stay that way, but there's a chance with mitre_components to leverage some of the work we've done elsewhere on MITRE enrichment.

[pyth0n1c]

Minor feedback - in line with latest PEP guidance, we should use
variable_name: None | TypeA
rather than
variable_name: Optional[None]

[ljstella]

After conversation w/ the folks that caused this PR to kickoff, looks like we should add some validation around the mitre_component portion to ensure that its name matches one of the x-mitre-data-component entities from ATT&CK. Validating that the field matches should prevent issues with bad data leaking downstream.

[ljstella]

After conversation this week, this should now be good to land in association with splunk/security_content#3287

Validations for it can come later when we have a better idea of how we might want to use it (likely on research.splunk.com)