added flutter to t2 sde(build from source)

[mitanjan]

• Adds a new flutter package to T2 SDE that builds the complete Flutter engine from source using Google's depot_tools (gclient + ninja)
• Uses gclient to fetch the Flutter engine tree at the tagged release version, with unnecessary sub-deps disabled (Android, Fuchsia, esbuild, Linux sysroot) and ANGLE's astc-encoder fetched via custom_deps to work around depot_tools recursedeps limitations
• Builds the engine with use_default_linux_sysroot=false so it links against the host T2 system libraries; applies patches for X11/xcb header paths, ANGLE vulkan-deps symlink, and DisplayVkXcb.cpp Bool/Status macro conflicts

Fixes

• System gn wrapper (/usr/bin/gn calls python3 gn.py) is not usable as a CIPD gn replacement; only substitute the native-compiled gn binary when available

• engine-dart-sdk.stamp must be sourced from bin/internal/engine.version, not bin/cache/engine.stamp (the latter is created by the flutter tool at first run and is absent during package build)

[rxrbln]

as mentioned in #303 new code in .desc please. this looks rather excessive, any change this could be done a bit more minimal? thanks!

[mitanjan]

Hello, Doc @rxrbln i have included all the codes in .desc file. The build is working fine. thanks!