[Feature Request] Handling optional parts in date format

Hello,

I think It could be a nice feature to handle optional parts in date format with `[]`
For example in my case, I can have DateTime with second fractions or not (it's adaptive).

Actually, I need to perform a `Substr` :

```
FromDateTime(Substr(JSONPATH(payload, '$.created_at'), 0, 18), 'yyyy-MM-dd''T''HH:mm:ss')
```

But It could be handled like that :

```
FromDateTime(JSONPATH(payload, '$.created_at'), 'yyyy-MM-dd''T''HH:mm:ss[.SSS]''Z''')
```

It's already provided by Java 8 and [DateTimeFormatter](https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html), in documentation we have :

```
[ optional section start
] optional section end
```

Start by locating the FromDateTime date-format handling and review how its current formatter processes the supplied pattern. The feature is complete when a pattern with an optional [.SSS] section accepts timestamps both with and without second fractions, while preserving the existing behavior.

Written by the indexing model from the issue text.