Allow passing instance method or conditional expressions to option ignore_serialize on JSON::Field

[cyangle]

Closes #11803

Allow passing instance method or conditional expressions to option ignore_serialize on JSON::Field from #11803.

It seems to be trivial to implement since the compiler and stdlib are written in crystal itself.

The new specs provide example usages:

• Use an instance method to decide whether to ignore the field
• Dynamically emit nulls based on @{{key}}_present

[straight-shoota]

LGTM 👍

[vlazar]

@cyangle Was it not possible to make existing ignore_serialize option to do what it did before + new behavior introduced here?

[cyangle]

@cyangle Was it not possible to make existing ignore_serialize option to do what it did before + new behavior introduced here?

Great idea, let me see if I could make it backward compatible or not.

[cyangle]

@straight-shoota

In last commit, I implemented what @vlazar suggested.

It works and should be backward compatible.

I don't like the nested unless branches though, suggestions are welcome.

[straight-shoota]

At first I thought having two separate fields would be better. But I see it's probably a good idea to merge them.
I just have a few comments to simplify the implementation.

[cyangle]

@straight-shoota Could this PR be merged before the 1.4 release?

[straight-shoota]

We need another approval for this. @beta-ziliani maybe?