Introduce mTLS support in the client stack

[kevinchalet]

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Describe the solution you'd like

Pro Santé Connect is officially adopting the standard version of mTLS and plans to make it mandatory later this year: #2172.

To ensure our PSC integration keeps working once this breaking change is adopted, we'll need to add tls_client_auth support to the OpenIddict client stack.

Additional context

Note: we'll also update the validation stack to support mTLS for introspection requests as part of this task.