Proposal: apply Kotlin default arguments for @RequestParam#defaultValue

[xuanswe]

Similar to @RequestParam#required, which is determined by Kotlin type automatically, I would propose to support Kotlin default arguments for @RequestParam#defaultValue.

For example, @RequestParam(defaultValue = "1") size: Int could be just @RequestParam size: Int = 1.

[sdeleuze]

Looks like a duplicate of #21139.