plain index needs autodetection of date formats for timestamp fields

[tomatolog]

Proposal:

as the #1744 added autoconversion of string into timestamp attribute on posting data into RT index the plain index data sources also need such conversion

Checklist:

^{To be completed by the assignee. Check off tasks that have been completed or are not applicable.}

• Task estimated
• Specification created, reviewed and approved
• Implementation completed
• Tests developed
• Documentation updated
• Documentation proofread
• Changelog updated
• OpenAPI YAML updated and issue created to rebuild clients