feat(Authentication): Add support for Client Assertion

[evansims]

Description

This PR adds support for Client Assertion, as an alternative to using Client Secrets, when making requests against the Authentication API.

Changes include the introduction of two new SDK configuration properties:

• clientAssertionSigningKey, which accepts an OpenSSLAsymmetricKey object, a PEM formatted private key as a string, a file path to a PEM certificate (e.g. file://path/to/file.pem), or null to disable the feature. Defaults to null.
• clientAssertionSigningAlgorithm, which accepts a string. Defaults to RS256.

When configured, the clientAssertionSigningKey will take precedence over any configured clientSecret, where appropriate.

When passing a string as the value to clientAssertionSigningKey, the format can be anything the OpenSSL openssl_pkey_get_private()

Changes also include a new class, Auth0\SDK\Token\ClientAssertionGenerator. This is a template interstitial for Auth0\SDK\Token\Generator that will apply the necessary configuration to create a JSON Web Token appropriate for use with the Client Assertion feature.

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist

• [x] My code follows the contributing guidelines of this repo
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass with my changes
• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 code of conduct

[github-actions[bot]]

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[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (87a7de7) compared to base (2e23ec4). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #699 +/- ## =========================================== Coverage 100.00% 100.00% Complexity 1282 1282 =========================================== Files 66 66 Lines 4498 4498 =========================================== Hits 4498 4498 ``` | [Impacted Files](https://codecov.io/gh/auth0/auth0-PHP/pull/699?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [src/API/Authentication.php](https://codecov.io/gh/auth0/auth0-PHP/pull/699?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#diff-c3JjL0FQSS9BdXRoZW50aWNhdGlvbi5waHA=) | `100.00% <100.00%> (ø)` | | | [src/Configuration/SdkConfiguration.php](https://codecov.io/gh/auth0/auth0-PHP/pull/699?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#diff-c3JjL0NvbmZpZ3VyYXRpb24vU2RrQ29uZmlndXJhdGlvbi5waHA=) | `100.00% <100.00%> (ø)` | | | [src/Token/Generator.php](https://codecov.io/gh/auth0/auth0-PHP/pull/699?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#diff-c3JjL1Rva2VuL0dlbmVyYXRvci5waHA=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0)

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