Feature : add EnumFeature.WRITE_ENUMS_TO_LOWERCASE

[JooHyukKim]

Summary

• resolves new feature Allow serializing enums to lowercase (EnumFeature.WRITE_ENUMS_TO_LOWERCASE) #3053 Fix #3053 (Serialize enums to lowercase) #3054
• mostly done by @knyttl whom I can't seem to tag.
• I only added one more test and replaced the Enum Config to EnumFeature class

[cowtowncoder]

Thank you @JooHyukKim ! Looks good -- but I realized something reading through it... method isEnabled(DatatypeFeature) SHOULD be accessible via MapperConfig but wasn't yet.
So I went ahead and changed this (see 6c8c151) so it is now there.

So I think you can simplify code by keeping old method signature & not having to deprecate old method.

[JooHyukKim]

Thank you @JooHyukKim ! Looks good -- but I realized something reading through it... method isEnabled(DatatypeFeature) SHOULD be accessible via MapperConfig but wasn't yet. So I went ahead and changed this (see [6c8c151](https://githu.......

@cowtowncoder Good call! How could I not think of that.

So I think you can simplify code by keeping old method signature & not having to deprecate old method.

Done and tested. 🙏🏼🙏🏼