README.md: List all unimplemented API functions

[wprzytula]

All functions present in cassandra.h that are not implemented in the CPP-Rust Driver are now listed in the README.md file.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• PR description sums up the changes and reasons why they should be introduced.
  - [ ] I have implemented Rust unit tests for the features/changes introduced.
  - [ ] I have enabled appropriate tests in Makefile in {SCYLLA,CASSANDRA}_(NO_VALGRIND_)TEST_FILTER.

[Copilot]

Pull Request Overview

This PR enhances the README by clarifying the limitations note and adding a comprehensive list of unimplemented Cassandra API functions.

• Refines the “Limitations” note wording for clarity
• Introduces a new “Unimplemented functions from cassandra.h” section
• Enumerates missing driver functions by API group

Comments suppressed due to low confidence (2)

README.md:303

• Duplicate entry 'cass_function_meta_body' appears twice under CassFunctionMeta. Remove the redundant line to avoid confusion.

- cass_function_meta_body

README.md:286

• [nitpick] Heading 'CassColumn' is missing a colon suffix for consistency with other section headings. Add ':' after the heading.

#### CassColumn

[Lorak-mmk]

I think we should list reasons for why something is unimplemented. Some (maybe not all) possible reasons:

• Planned, not yet done
• Functionality not present in Scylla
• Deprecated in cpp-driver
• Deprecated and noop in cpp-driver

[wprzytula]

I think we should list reasons for why something is unimplemented. Some (maybe not all) possible reasons:

* Planned, not yet done

* Functionality not present in Scylla

* Deprecated in cpp-driver

* Deprecated and noop in cpp-driver

Agreed. I'll do that in a follow-up, OK?