[FEATURE] Support reading/writing to global variables in firmware

**Is your feature request related to a problem? Please describe.**
I want support for reading/writing to global variables in the firmware shared lib.

**Describe the solution you'd like**
Similar to tagged data, there should be a simple interface for accessing/modifying global variables from the test code.

So test code provides variable name -> sim core fetches data from loaded object -> sim core returns data as Python object

Things to consider:
- We'll need some way to fetch the address/size of data by symbol name (Also not sure how simple it is to support array/struct accesses)

No files or tests are named. Start by tracing the firmware shared library's existing tagged-data interface and how test code communicates with the loaded object. Define how symbol names map to addresses and sizes, including the expected handling of arrays and structs, then verify that values can be returned as Python objects and written back.

Written by the indexing model from the issue text.