Explain how to generate a `SIGNING_KEY` in `EncryptionKeyWasInvalid`

[TimWolla]

Description

The exception message in EncryptionKeyWasInvalid could suggest php tempest key:generate.

tempest-framework/packages/cryptography/src/Encryption/Exceptions/EncryptionKeyWasInvalid.php

Line 19 in 743e8d8

return new self('The encryption key is missing or empty. Ensure you have a `SIGNING_KEY` environment variable.', $algorithm);

Similarly the becauseLengthMismatched() option could suggest the number of characters the key had, since this is not trivial to match up due to the base64_decode() step involved during config parsing.

Benefits

Make it easy for folks to generate a valid encryption key without them resorting to possibly unsafe options.