[FEATURE]: opportunity to pass custom `datetime` format for Java/Kotlin generation

[l3r8yJ]

It would be very suitable to add new flag for Java/Kotlin generation --datetime-format MY-CUSTOM-FORMAT

Context (Input, Language)

Input Format: flag for cli - --datetime-format
Output Language: Java, Kotlin

Description

Very often we need to provide custom datetime format; In case when automatically generated classes are published as standalone artifact (jar library for nexus). We can't change them at this moment.

Current Behaviour / Output

Not possible

Proposed Behaviour / Output

Inside generated classes would be custom format for datetime

    private static final DateTimeFormatter DATE_TIME_FORMATTER = new DateTimeFormatterBuilder()
            .appendOptional(DateTimeFormatter.ISO_DATE_TIME)
            .appendOptional(DateTimeFormatter.ISO_OFFSET_DATE_TIME)
            .appendOptional(DateTimeFormatter.ISO_INSTANT)
            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.SX"))
            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ssX"))
            .appendOptional(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"))
            .appendOptional(DateTimeFormatter.ofPattern("MY-CUSTOM-FORMAT")) // adds own format
            .toFormatter()
            .withZone(ZoneOffset.UTC);

Context

I wrapped quicktype cli into gradle plugin, that generates Java classes and publishes them as artifacts that can be added to project as a dependency. Now I can't provide custom date time format in config. I see bad workaround how i can do this, but it's seems inconvenient to me (need to parse generated classes and insert new line in every generated class, which is too ugly).

[l3r8yJ]

@schani can you take a look, please?