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]