add template userfunc for timestamp, with configurable format

[mimir-d]

New feature: for usage in the test descriptor template json, add a new userfunc that takes the current server timestamp and renders it in the given format spec, as a string.

Proposal:

"param_formatted": "log_{{ timestamp '%H:%m:%s' }}",
"param_unix_stamp_by_default": "log_{{ timestamp }}"