[REQ][Kotlin] Add spring webclient library

[DSniatecki]

I suggest adding spring webclient as a library for kotlin generator.

Webclient is often used with spring boot and if there is kotlin-spring generator, there should be possibility to generate kotlin client with webclient library.

[Sekir9]

I suggest adding spring webclient as a library for kotlin generator.

Webclient is often used with spring boot and if there is kotlin-spring generator, there should be possibility to generate kotlin client with webclient library.

Good idea, that would be very helpful

[stefankoppier]

This would be good addition. I'd like to give this a shot.