Fix REST Client parameter field dropdowns

[gvdutra]

Fixes #7530.

Summary

• keep separate column definitions for query and matrix parameter fields
• populate both parameter field dropdowns from incoming fields
• add a regression test for both dropdowns
• clarify query and matrix parameter configuration in the REST Client documentation

Root cause

The query and matrix parameter tables shared the same ColumnInfo array field. Initializing the matrix parameter tab replaced the query parameter column reference, so field discovery updated only the matrix parameter dropdown.

Validation

• ./mvnw -pl plugins/transforms/rest -Dtest=RestDialogTest test
• ./mvnw -pl plugins/transforms/rest -DskipTests spotless:check
• git diff --check