keys: rename show to secret

[leighmcculloch]

What

Rename keys show to keys secret.

Why

Close #1804

[willemneal]

Perhaps private? We are renaming address to public

[Ifropc]

I think secret is better than private because we usually use "secret key" instead of "private key" across our docs/sdks/etc.

[leighmcculloch]

Private is historically the more common term in asymmetric key management, but some libs, such as the libs in the Rust ecosystem, have used "Secret". "Secret" contrasts the key types in key pairs more clearly, and avoids problems where they are easily confusable, such as "pk" can mean private or public key.

Note that in SDKs like the js-stellar-sdk the secret terminology is used too.