Remove ExtensionGrantValidator

[trmartin4]

Objective

@coroiu and I discussed this, and prior to merging the fido2 branch to master, we would like to remove the ExtensionGrantValidator and references to it from the branch. This validator will be replaced by the WebAuthnGrantValidator (and webauthn grant type) in the final implementation, so we did not want to leak this to master only to be replaced, especially since it exposed a new potential authentication method.

This will be the final change before we can merge the fido2 branch to master. The fido2 branch will contain the (feature-flagged) changes to enable passkey registration.

Code changes

• ExtensionGrantValidator.cs: Removed validator.
• ApiClient.cs: Removed extension grant type.
• ServiceCollectionExtensions.cs: Removed DI for ExtensionGrantValidator
• openid-configuration.json: Removed extension grant type.

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