[Feature Request] Support PFX certificate authentication

[jiasli]

MSAL client type

Confidential

Problem Statement

MSAL currently requires PEM certificate for ConfidentialClientApplication:

microsoft-authentication-library-for-python/msal/application.py

Lines 226 to 228 in 52b1fc5

	MSAL Python requires a "private_key" in PEM format.
	If your cert is in a PKCS12 (.pfx) format, you can also
	`convert it to PEM and get the thumbprint <https://github.com/Azure/azure-sdk-for-python/blob/07d10639d7e47f4852eaeb74aef5d569db499d6e/sdk/identity/azure-identity/azure/identity/_credentials/certificate.py#L101-L123>`_.

The conversion logic is deferred to downstream application, such as Azure Identity library.

Proposed solution

MSAL itself can support PFX certificate authentication so that other clients such as Azure CLI can also benefit from it.

See Azure/azure-cli#28699

[rayluo]

See also the first comment in #685

[rayluo]

Documented in the last paragraph of this parameter.