"to_timestamp" should recognize TIMESTAMP_ISO8601 grok-pattern

[Djeezus]

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Use Cases

"to_timestamp" does not recognize TIMESTAMP_ISO8601 pattern/format is. "parse_grok" honors TIMESTAMP_ISO8601 pattern, but "to_timestamp" does not.

I would suggest to recognize the same pattern in both "to_timestamp" and "parse_grok"-patterns, so that a .timestamp can be matched and extracted from any field without the need to use parse_timestamp custom formats when the parse_grok did already match on an existing pattern.

Attempted Solutions

In VRL tested:

$ . = { "@timestamp": "2023-01-10T05:05:05.55Z", "@metadata": { "beat": "filebeat", "type": "_doc", "version": "7.9.2" }, "host": { "name": "somehostname" }, "message": "2023-01-10 10:03:30.745 blah blah blah blah" }
{ "@metadata": { "beat": "filebeat", "type": "_doc", "version": "7.9.2" }, "@timestamp": "2023-01-10T05:05:05.55Z", "host": { "name": "somehostname" }, "message": "2023-01-10 10:03:30.745 blah blah blah blah" }

$ .flat = parse_grok!(.message, "^%{TIMESTAMP_ISO8601:datestamp} %{GREEDYDATA:logline}")
{ "datestamp": "2023-01-10 10:03:30.745", "logline": "blah blah blah blah" }

$ .timestamp = to_timestamp!(.flat.datestamp)
function call error for "to_timestamp" at (13:43): No matching timestamp format found for "2023-01-10 10:03:30.745"

$ .timestamp = to_timestamp!(.@timestamp)
t'2023-01-10T05:05:05.550Z'

### Proposal

_No response_

### References

#15802

### Version

v0.26.0