Introduced a new `DocumentService` class to handle fetching
PDF document bytes from an API endpoint. The service uses
`HttpClient` for HTTP communication and `IOptions<ApiOptions>`
for accessing API configuration. Added the `GetDocumentAsync`
method to perform the HTTP GET request, handle responses, and
return the document bytes along with the HTTP status code.
Included necessary `using` directives and encapsulated the
service in the `EnvelopeGenerator.ReceiverUI.Services` namespace.
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