Allow external radio types

[puddly]

This will need a corresponding PR to Core but will allow for pre-alpha radio libraries (Ziggurat, ZBOSS, BLZ, and ESPZB) to be used in Home Assistant:

# A custom component, unfortunately, to manage dependencies
zha_custom_radio_libraries:
  - "git+https://github.com/zigpy/ziggurat"

zha:
  external_radio_libraries:
    - radio_type: ziggurat
      display_name: Ziggurat
      module_path: zigpy_ziggurat.zigbee.application:ControllerApplication
      description: An experimental open source Zigbee stack implemented in Rust.

This is fundamentally a development tool so expect breakage if you rely on it 😄.

[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.88%. Comparing base (a2de928) to head (0a08169).
⚠️ Report is 140 commits behind head on dev.

Files with missing lines	Patch %	Lines
zha/application/gateway.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #461      +/-   ##
==========================================
- Coverage   96.88%   96.88%   -0.01%     
==========================================
  Files          63       63              
  Lines       10361    10355       -6     
==========================================
- Hits        10038    10032       -6     
  Misses        323      323              

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[puddly]

I've slightly expanded the scope of this PR to clean up and entirely remove the RadioType enum. Instead, we can have structured information within the Gateway object accessible via an API call.

[fangzheli]

Thanks @puddly for working on this — external radio type support would be a big deal for zigpy-blz.

We currently maintain a custom ZHA component to patch BLZ radio support into ZHA, which requires tracking every HA Core release. This PR would let us drop that entirely and ship as a standalone zigpy-blz package (now on PyPI).

We tested this PR locally with a Third Reality BL702 dongle — the Gateway successfully resolves and connects to zigpy-blz through external_radio_libraries. Happy to help with further testing as this moves forward.