Add OpenSSL::Digest.digests to get a list of available digests #982

Job	Run time
openssl-1.0.2u	2m 35s
openssl-1.1.0l	1m 31s
openssl-1.1.1w	1m 44s
openssl-3.0.13	2m 43s
openssl-3.1.5	3m 4s
openssl-3.2.1	3m 22s
libressl-3.1.5	2m 11s
libressl-3.2.7	2m 14s
libressl-3.3.6	2m 15s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 17s
libressl-3.6.3	2m 16s
libressl-3.7.3	2m 19s
libressl-3.8.2	2m 11s
openssl-3.0.13 fips	2m 48s
openssl-3.1.5 fips	2m 53s
openssl-3.2.1 fips	3m 9s
openssl-head	3m 45s
openssl-head fips	3m 24s
openssl-head no-legacy	3m 21s
ruby-versions / Generate Ruby versions	6s
ubuntu-22.04 2.7	58s
ubuntu-22.04 3.0	52s
ubuntu-22.04 3.1	56s
ubuntu-22.04 3.2	58s
ubuntu-22.04 3.3	54s
ubuntu-22.04 head	58s
ubuntu-22.04 truffleruby	1m 32s
ubuntu-22.04 truffleruby-head	1m 28s
ubuntu-20.04 2.7	58s
ubuntu-20.04 3.0	55s
ubuntu-20.04 3.1	53s
ubuntu-20.04 3.2	54s
ubuntu-20.04 3.3	52s
ubuntu-20.04 head	52s
ubuntu-20.04 truffleruby-head	1m 24s
macos-latest 2.7	1m 25s
macos-latest 3.0	1m 17s
macos-latest 3.1	55s
macos-latest 3.2	1m 9s
macos-latest 3.3	1m 41s
macos-latest head	1m 20s
macos-latest truffleruby-head	1m 58s
windows-latest 2.7	5m 56s
windows-latest 3.0	4m 18s
windows-latest 3.1	3m 54s
windows-latest 3.2	4m 14s
windows-latest 3.3	5m 5s
windows-latest ucrt	4m 1s
windows-latest mswin	3m 4s
	1h 48m 3s

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