Add support for v5 keys/signatures. #3299

Job	Run time
cmake-offline-googletest	4m 4s
cmake-offline-googletest-src	4m 11s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 17s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 10s
package-source	33s
system-sexpp, sexpp shared libs on, rnp shared libs on	2m 42s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	4m 39s
system-sexpp, sexpp shared libs on, rnp shared libs off	3m 18s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 10s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 15s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 39s
system-sexpp, sexpp shared libs off, rnp shared libs off	2m 43s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 11s
package	51s
debian-tests	10s
	46m 53s

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