[ML] Allow "no timestamp" to be specified as the timestamp format in file upload

[droberts195]

In elastic/elasticsearch#90764 we added the ability to specify null as the timestamp format override in the file structure finder endpoint.

The file data visualizer/file upload in Kibana can now be enhanced to make this option available. If a user chooses to add overrides then specifying "there is no timestamp" should be an alternative to overriding the timestamp format and if the user selects this then timestamp_format=null should be passed as an argument the next time the file structure finder endpoint is called.

Relates to #63526

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[jgowdyelastic]

@droberts195

When analysing a semi structured text file with no timestamps, I get an error saying no timestamp could be found. If I then run analysis again, setting timestamp_format to null it works correctly.
The original error is

[illegal_argument_exception: [illegal_argument_exception] Reason: Could not find a timestamp in the sample provided]: Could not find a timestamp in the sample provided
which makes me wonder why the original analysis can't perform a check to see if using timestamp_format: null produces good results.

As it is now, the UI user has to manually run the analysis again. The UI could look out for this error and attempt a run with timestamp_format: null automatically, but it would be faster if es did this instead.

[droberts195]

The UI could look out for this error and attempt a run with timestamp_format: null automatically, but it would be faster if es did this instead.

The problem is, 9 times out of 10 a semi-structured log file will have a timestamp but we just didn't detect it. Assuming each line in the file is a separate message will always work, but will look very stupid to a user who can see the timestamp in a format that makes complete sense to them but was missed by our timestamp detection.