Adds support for mutual TLS

[jonerickson]

The need for this PR arose out of our use of this package in a SaaS we are developing where we will pushing data via web hooks that is extremely sensitive in nature. We need to use all available options to ensure the integrity of our data and so we are requiring mutual TLS authentication. This PR adds support for mutual TLS that is already natively supported in Guzzle.

A custom CA cert can be optionally passed to the verifySsl method. The method now accepts booleans or strings to support cert paths.

Implementation:

WebhookCall::create()
  ->mutualTls(
    certPath: storage_path('path/to/cert.pem'), 
    certPassphrase: 'optional_cert_passphrase', 
    sslKeyPath: storage_path('path/to/key.pem'), 
    sslKeyPassphrase: 'optional_key_passphrase'
  )
  ->verifySsl(storage_path('path/to/ca.pem'))
  ->dispatch();

[freekmurze]

Thank you!