New rule: toString over a nullable value

[BraisGabin]

Expected Behavior of the rule

val a: Any?
val key: String = a.toString() // this line should be flagged
val key2: String = "$a" // same here

This code compiles, and have a defined behaviour. But, usually, it is a potential bug. The idea is to have a rule that detects the use of toString() (explicit or implicit) and flag it.

Context

It's really strange that you really want to print "null" when you have a null value. If you are working with UI you probably want "", "-" or something else. And the same works for any other piece of code for example this one: https://github.com/detekt/detekt/pull/2898/files#diff-6576486bfd0ee34b1da2a5967fba6cf4R27