(WIP) FFI: add function to retrieve details on signature validation errors. #3927

Job	Run time
cmake-offline-googletest	5m 9s
cmake-offline-googletest-src	4m 58s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 2s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 47s
system-sexpp, sexpp shared libs on, rnp shared libs off	3m 9s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	5m 45s
system-sexpp, sexpp shared libs off, rnp shared libs on	3m 1s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	1m 36s
system-sexpp, sexpp shared libs off, rnp shared libs off	3m 16s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	1m 6s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	47s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 47s
package-source	40s
package	56s
debian-tests	18s
	43m 17s

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