Add "variable initialization" config element

### Use case

* Initialize variables "at the beginning of the test plan"
* Initialize variables before using in pre/post-processors

### Possible solution

Add `Variable Initialization` config element so the users can init variables.
It might be that "initial variable initialization" might need a different controller as it should be illegal to use "undefined" variables for the variable initializers expressions.

See also:
* https://github.com/apache/jmeter/issues/4360

### Possible workarounds

_No response_

### JMeter Version

5.5

### Java Version

_No response_

### OS Version

_No response_