fix(privacy): Re-compile engines from cached combined rules when failing to deserialize cached engine on iOS

[StephenHeaps]

• This change only affects the flow where we fail to deserialize the AdblockEngine using the serialized list.dat from our cache.
  • When this occurs for the standard engine, we will use instead re-compile the AdblockEngine from the cached combined rule lists stored at /Caches/engines/<engine_type>/list.txt (we combine rule lists and cache them), then re-caches the engine to the serialized DAT format.
• This recompile comes with a small one-time performance hit (~1.2s to compile from plaintext during startup vs ~0.6s to compile from DAT on iPhone XS w/ A12 Bionic), as compiling the adblock engine from plaintext is slower than loading from serialized data file.
• Adds a adblock debug option to 'corrupt' this cache by writing empty data file to the cache location, which causes us to fail to deserialize the engine on next launch.

Resolves brave/brave-browser#46545

1. In the QA menu / developer options menu, open the AdBlock Debugger section, then tap Corrupt Adblock Engine DAT Caches
2. After successful alert, restart the app (if a developer, restart with debugger attached to view console logs)
3. (Developers only) On launch you should see Set <engine_type> (txt) in console logs to indicate the engine being compiled from txt
4. Verify adblock engine is working as expected (ex visit a website and confirm >0 items in shields panel)
5. Restart the app
6. (Developers only) On launch you should see Set <engine_type> (dat) in console logs to indicate the engine being loaded from cached DAT again
7. Verify adblock engine is working as expected (ex visit a website and confirm >0 items in shields panel)

[brave-builds]

Released in v1.81.44