Added an option to render Kotlin functions with suspend modifier

[thevietto]

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Description

Added a configuration option for Kotlin generator to render functions with suspend modifier:

when disabled:

fun sayHello(): String

when enabled:

suspend fun sayHello(): String

I know this could be solved by providing a custom Freemarker template to the config, but it seems like supporting Kotlin coroutines is something one would expect to get out of the box.

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Changes were made to:

• Codegen library - Java
• Codegen library - Kotlin
• Codegen library - Scala
• Maven plugin
• Gradle plugin
• SBT plugin

[jxnu-liguobin]

It looks good, looking forward to you completing it. By the way. SBT cannot compile kotlin by default unless using third-party plugins. So you don't need to consider SBT. And I am considering removing the code related to kotlin from the sbt plugin in this library.

[thevietto]

It looks good, looking forward to you completing it. By the way. SBT cannot compile kotlin by default unless using third-party plugins. So you don't need to consider SBT. And I am considering removing the code related to kotlin from the sbt plugin in this library.

thanks! Yeah I didn't touch the SBT module
codestyle is fixed, should be good now

[kobylynskyi]

Looks good! Thanks for the contribution.