FIR: introduce serializer extension base for protocol-based platform extensions

[jsjeon]

One day, if fir is used for other platforms (as well as JVM), we may want the serializer extension to deal with annotation serializations in a centralized way, similar to how SerializerExtensionBase (under :compiler:serialization) does.

[mglukhikh]

Guys, sorry, now I don't understand why this FirKotlinSerializerExtensionBase is introduced here and how do you want to use it. As far as I understand, at this moment the class will be unused.

[udalov]

Indeed, it doesn't make much sense to introduce this class if we don't intend to add support for JS or Native under FIR in the nearest future. But if there are such plans, then it's required :).

[mglukhikh]

Well, I don't know, but I don't expect JS or Native support will be added until August or September. @semoro , what do you think about these perspectives?