feat: add external async signing and encryption strategies

[troyfactor4]

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Description

Allow the user of the library to specify a signing or encryption strategy externally. Useful when signing keys are in remote servers or HSM devices and signing must happen async or with the use of custom APIs.

Testing

• [ x ] This change adds test coverage for new/changed/fixed functionality

Checklist

• [ x ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [ x ] All active GitHub checks for tests, formatting, and security are passing
• [ x ] The correct base branch is being used, if not master

[troyfactor4]

Obsoletes #64
Closes issue #63

[troyfactor4]

bump - any feedback here on potential changes? Hoping I followed the correct process for MR. I feel like async signing over PKCS11 could really improve the security posture of some organisations.