Add basic secret key sanity checks #1037

Job	Run time
Check Examples	41s
Docs	36s
format	18s
clippy (default-tls)	59s
clippy (rustls-tls-webpki-roots)	52s
clippy (rustls-tls-native)	58s
Test Clients (default-tls)	51s
clippy (async-std-surf)	1m 7s
Stripe Mock Tests (default-tls)	57s
Test Clients (rustls-tls-webpki-roots)	54s
verify-codegen-crate	1m 44s
Stripe Mock Tests (rustls-tls-webpki-roots)	57s
Test Clients (rustls-tls-native)	56s
Stripe Mock Tests (rustls-tls-native)	54s
Test Clients (async-std-surf)	1m 6s
	13m 50s

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