Calling Signal entity HTTP API fails with content-type=application/json

### Context
In the [http api documentation for signalling a durable entity](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-http-api#signal-entity) it is stated that the request content (payload) should be JSON-formatted, implying that the Content-Type should be application/json.
Calling the API using this Content-Type=applicaton/json will throw an error in the get_input() function of the Durable entity context with the error "the JSON object must be str, bytes or bytearray, not dict"

### How to reproduce
Call the API using Content-Type = application/json

### Workaround
Call the API using Content-Type = text/plain

Start by tracing the Signal entity HTTP API handling to the Durable entity context's get_input() function, using the documented signal-entity request and the reported Content-Type behavior as the reproduction. Verify the JSON payload is accepted with application/json, while preserving the existing text/plain workaround behavior, and add or update coverage for both request forms if the surrounding tests provide a suitable entry point.

Written by the indexing model from the issue text.