Add persisting RSA keys #2919

Job	Run time
clang-sanitizers (,thread)	2m 26s
clang-sanitizers (,address,undefined)	2m 32s
linux-byo-crypto	1m 50s
linux-compiler-compat (clang-3)	2m 12s
linux-debug	1m 45s
linux-compiler-compat (clang-6)	2m 10s
linux-compiler-compat (clang-8)	2m 10s
downstream	2m 14s
linux-shared-libs	1m 50s
linux-compiler-compat (clang-9)	2m 12s
windows	4m 1s
linux-compiler-compat (clang-10)	2m 11s
windows-debug	4m 13s
linux-compiler-compat (clang-11)	2m 11s
windows-vc14 (x86)	2m 39s
linux-compiler-compat (gcc-4.8)	1m 59s
windows-vc14 (x64)	3m 11s
linux-compiler-compat (gcc-5)	2m 8s
linux-compiler-compat (gcc-6)	1m 58s
linux-compiler-compat (gcc-7)	1m 55s
linux-compiler-compat (gcc-8)	2m 6s
freebsd	2m 39s
macos	44s
macos-debug	29s
macos-x64	41s
openbsd	5m 57s
windows-app-verifier	4m 46s
windows-shared-libs	4m 9s
linux-compat (manylinux1-x64)	1m 57s
linux-compat (manylinux1-x86)	1m 59s
linux-compat (manylinux2014-x64)	2m 49s
linux-compat (manylinux2014-x86)	1m 55s
linux-compat (fedora-34-x64)	1m 48s
linux-compat (opensuse-leap)	2m 14s
linux-compat (rhel8-x64)	1m 59s
linux-compat (al2-x64)	1m 43s
	1h 25m 42s

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