Support for request-level variables with caching

[stefaanMLB]

It would be great to have variables at the request level that cache the values that were used before Basically, add a list the request window where on can add variables to be user in that request's url, headers or body and add a value drop-down test field in which new values can be set or recent variables can be chosen Having the cache synchronised between equally-named variables in different requests would be a great extention.

DBeaver's "where clause editor field" can server as inspiration