Improve testing

Merge pull request #5 from herodez/feature/php8-support

feat: Add support for php8

Sign or not sign on Encrypt?

Added a new parameter for encrypt and encryptFile to allow control if sign is required or not.
By default the behaviour is the same before add parameter.

Argument validations

• Added exceptions on argument failures

Valentine's gift

• Implemented decryptFile
• Added fingerprint to the listKeys method
• Escaped shell arguments and command
• Implemented better test coverage
• Updated PHPUnit dependencies