Add `java.time.Clock` converter

[GrahamBorland]

Adds a function to convert a java.time.Clock to a kotlinx.datetime.Clock.

[ilya-g]

Could you describe cases when such conversion could be useful?

[GrahamBorland]

When you have a Java clock (e.g. injected from DI on Android) and you want to pass it to a KMP library using kotlinx-datetime.