ash: Hide provisional extensions behind new `provisional` feature flag #3081

Job	Run time
Check ash, ash-window and ash-rewrite MSRV (1.69.0)	44s
Check	42s
Generated	35s
Clippy (x86_64-pc-windows-msvc)	43s
Build-test docs	47s
Clippy (x86_64-unknown-linux-gnu)	54s
Rustfmt	6s
Clippy (x86_64-apple-darwin)	46s
Test Suite	1m 45s
Clippy (aarch64-apple-ios)	45s
	7m 47s

Provide feedback