feature: add support for object input default values and add support for default inputs to kotlin2

[dwilkolek]

• Added support for object default input param to kotlin and java input types
• Implemented default input params for kotlin2 generator

closes #297

[srinivasankavitha]

The changes look good to me. Would you also be able to fix the Java code path as well as part of this PR? If not, we can open a new issue for it.

[dwilkolek]

Sure, I'll take a stab at it shortly.

[dwilkolek]

While peeking at java code path I've found a bug in kotlin path. When using typeMapping with Person to DifferentlyNamedClass mapping then it would fail.

Regarding Java path it will be slightly more difficult. In Kotlin path I used named parameters. In Java I see 4 paths:

1. create json object and use ObjectMapper to create object in place
2. use default no param constructor and call setters in initialiser block
3. use constructor with arguments but it will require the same order and matching constructor for mapped types
4. use builder - this would be great for generated classes, but will most likely fail for type mapped classes.

[dwilkolek]

I've implemented option 2. Lmk what do you think 🙏

[dwilkolek]

I realised that kotlin2 did not support default values at all. Now experience with default values should be the same across codegen configurations.

[dwilkolek]

@mbossenbroek This PR contains kotlin2 changes - added support for default values in input types 😉
@srinivasankavitha I've implemented java code path using initialiser block

If it make sense to you I can split changes into 2 PRs; one for object defaults, one adding kotlin2 default values

[mbossenbroek]

Looks good to me! 👍 I'll let @srinivasankavitha merge it cause there are other changes in there too