[ML] File Data Visualiser - Allow custom timestamp format

[markwalkom]

Describe the feature: It'd be great to be able to specify custom timestamp formats in the Override Settings section.

Describe a specific use case for the feature:
I had a CSV file with a timestamp like 1/31/2019 1:40pm, which isn't listed in the dropdown of possible date formats.

Having the ability to create your own custom notation would be useful for occasions like this.

[elasticmachine]

Pinging @elastic/ml-ui

[droberts195]

This is the second checkbox in elastic/elasticsearch#38086. After that's implemented on the backend it should be easy to add to the UI.

[droberts195]

The change to the backend (currently targeting version 7.3) will not allow completely arbitrary timestamp formats, but will allow formats build from a reasonable variety of components in any order - see the docs change in https://github.com/elastic/elasticsearch/pull/41948/files#diff-cf0bc7031119acc3f6c3de16748677e6 for what is proposed to be supported.

1/31/2019 1:40pm would then be M/d/yyyy h:mma, and all those letter groups will be supported.

[droberts195]

Argh, there's a complication. It seems that Java makes it extremely hard to parse lower case am/pm (see https://stackoverflow.com/a/46480738).

So M/d/yyyy h:mma will be make it possible to parse 1/31/2019 1:40PM, but sadly not 1/31/2019 1:40pm.

However, this is not really the fault of the file structure finder, as downstream functionality like ingest pipeline and mappings also suffers from this.

[droberts195]

#36880 will add support for a certain amount of custom timestamp formats, including 1/31/2019 1:40PM. But, as stated in the previous comment, 1/31/2019 1:40pm is more problematic as it is not supported by Java.

[droberts195]

#38868 covers what was part 4 of #36880, as that is not being implemented in #38727.