Added a new `GetEnvelopeReceiverOfReceiver` method in the
`EnvelopeReceiverController` to retrieve an envelope receiver
based on the provided `envelopeKey`. The method is secured
with the `Receiver` authorization policy and uses the
`_mediator.Send` method to process a `ReadEnvelopeReceiverQuery`.
Included the `Microsoft.Extensions.Options` namespace to
support potential configuration needs. Added XML documentation
placeholders for the new method.
TekH
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