Improvement: App Store Connect API private key validation

[priitlatt]

Check that App Store Connect API keys specified by --private-key argument for app-store-connect are valid PEM encoded private keys, as opposed to checking just key contents prefix as it used to be.
This change will remedy the unexpected failures that happen due to JWT generation using invalid private keys.

Other changes inclued here:

• Do not expose specified private key in error log. Instead just state that it is not a valid PEM encoded key.
• Log JWT cache revoking with debug level instead of info, as it is mosly useless noise in the logs.

Chagnes apply to actions:

• all app-store-connect actions.