Adding feature to allow a callable as the signing key

[andrewjroth]

I made modifications to the CertificateBuilder.build to allow for a callable to be used as the private key. This will allow downstream usage of a key that leverages an external process or custom code to calculate the signature while maintaining the core functionality of assembling the x509 certificate.

Example usage:
test_callable_key.py

I tried to figure out how to verify that the signature is valid, but couldn't find a way to test it that was super complicated.

While the example is mostly pointless, this feature could be useful when attempting to use a key maintained in an external key store, such as with a hardware security module (HSM), AWS KMS, or other key signing service.

I initially asked if this was possible in pyca/cryptography#5921, which I found to be extremely difficult to try to build in.

Thanks for your consideration!

[andrewjroth]

I realize I got a "all checks have failed" from the integration tests, but I believe those failures are because of misconfigured tooling that I can't fix.

[sodre]

@andrewjroth, sorry I did not notice you had created a similar PR #11.