add pluggable crypto provider #2818

Job	Run time
build-windows	1m 49s
build (ubuntu-latest, nightly)	46s
build (ubuntu-latest, stable)	17m 9s
Build rustls-cert-gen as a standalone package	28s
Clippy	4m 46s
Validate external types appearing in public API	1m 33s
Format (unstable)	13s
build (ubuntu-latest, stable 7 months ago)	49s
build (ubuntu-latest, beta)	49s
Check semver compatibility	1m 10s
Check MSRV	1m 53s
Documentation (stable)	2m 5s
Measure coverage	44s
Documentation (nightly)	1m 58s
build (macOS-latest, stable)	57s
	37m 9s

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