Add generics to HashTree and ListedHashTree declaraitons

### Use case

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

I think we should migrate to `HashTree implements Map>` 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_