GRC: allow variable expressions for enum fields

[AsriFox]

Description

Block properties with dtype: enum can now be controlled dynamically by variables or expressions. The evaluated expression is then checked against the enum's options.

Related Issue

Fixes #6870

Which blocks/areas does this affect?

GRC

Testing Done

dvb_enums.grc.txt

Checklist

• I have read the CONTRIBUTING document.
• I have squashed my commits to have one significant change per commit.
• I have signed my commits before making this PR
• My code follows the code style of this project. See GREP1.md.
• I have updated the documentation where necessary.
• I have added tests to cover my changes, and all previous tests pass.

[AsriFox]

Some parameters may benefit from not having this dynamic evaluation, like output_language or generate_options in Options block. This will probably require adding a new hidden dtype.

[willcode]

Added Needs Action based on the comment above. Should this be changed to draft?