Add generics to HashTree and ListedHashTree declaraitons

[vlsi]

Use case

Currently, HashTree implements Map<Object, HashTree>, however, when the user knows the key is TestElement, it requires casts.

I think we should migrate to HashTree<KeyType> implements Map<KeyType, HashTree<KeyType>> or something like that.
However, adding generics is not fully backward compatible, I suggest we do that in 6.0

Possible solution

No response

Possible workarounds

No response

JMeter Version

5.5

Java Version

No response

OS Version

No response