Improve Talk HPB validation check by checking the shared secret

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Is your feature request related to a problem? Please describe.
When configuring the Talk HPB in the Talk admin settings, a check is performed on the user-entered parameters. The check doesn't take into account the shared secret as it just runs a request to the api/v1/welcome endpoint, that doesn't require authentication. This can be misleading as the connection will fail with a wrong shared secret.

Describe the solution you'd like
Let the Talk HPB validation check to take into account the shared secret as well.

Describe alternatives you've considered

Additional context

Start with the Talk admin settings validation and the api/v1/welcome endpoint mentioned in the issue. Trace how the validation request is built and how the shared secret is passed, then verify that a wrong secret fails validation while valid Talk HPB parameters succeed.