[flutter_tools] Implement Configuration extension slice and flutter config integration (#191445)

## Description

This PR implements Step 4 of Flutter Tools Extensibility: the
Configuration extension slice and `flutter config` integration.

### Architectural Overview
1. **`package:flutter_tools_core`**:
- Defines core, platform-agnostic configuration models: `FeatureFlag`
and `ConfigOption`.
2. **`package:flutter_tools_extension`**:
- Defines the `ConfigurationExtension` interface and service contract
(`config.getTitle`, `config.getFeatureFlags`,
`config.getConfigurations`).
3. **`package:flutter_tools_extension_linux_prototype`**:
- Implements `LinuxConfigurationExtension` in the Linux prototype
extension.
4. **`packages/flutter_tools`**:
- Implements `ConfigurationExtensionClient` to query isolate extensions
over RPC.
- Implements `ExtensionArgParserMixin` to dynamically contribute CLI
options to `ConfigCommand`.
- Exposes `ExtensionManager.configurationExtensions` and integrates
active extension configuration settings and feature flags dynamically
into `flutter config` (`settingsText` and arg parsing).

## Related Issues
Part of https://github.com/flutter/flutter/issues/190692

## Tests
- Added
`packages/flutter_tools/packages/flutter_tools_core/test/config_test.dart`
- Added
`packages/flutter_tools/packages/flutter_tools_extension_linux_prototype/test/linux_config_test.dart`
- Added
`packages/flutter_tools/test/general.shard/extension_protocol/config_service_test.dart`
- Added
`packages/flutter_tools/test/commands.shard/hermetic/config_test.dart`
- Added tests in
`packages/flutter_tools/test/commands.shard/hermetic/tool_extensions_integration_test.dart`
- Added integration tests in
`packages/flutter_tools/test/integration.shard/tool_extensions_test.dart`