[ML] Fix custom timestamp override with dot-separated fractional seconds

[droberts195]

Custom timestamp overrides provided to the find_file_structure
endpoint produced an invalid Grok pattern if the fractional
seconds separator was a dot rather than a comma or colon.
This commit fixes that problem and adds tests for this sort
of timestamp override.

Fixes #44110

[elasticmachine]

Pinging @elastic/ml-core

[droberts195]

This is marked >non-issue even though it's a bug because it was both introduced and fixed in 7.3.0, so does not need to be release noted.

[jpountz]

I moved it to v7.3.1.

[droberts195]

Given that the current process (from the email with subject "Blockers and preparing for releases") is "For minor releases we will switch to a model where we produce a build candidate once per week up to the last which we will do 72 business hours before the release date" it seems to me that all this relabelling is just creating work. Someone will now have to change the label back when this is merged. Surely it would be easier if bug fixes merged well before that final build candidate for a minor were just labelled with that minor version?

[jpountz]

I agree it is extra work. I tend to like the fact that it gives us more flexibility with moving dates and doesn't require everyone to follow the release process too closely. I'll start a discussion.

[davidkyle]

LGTM