Fix incomplete reads in read_secure_message() by properly handling partial TCP payloads && README updates

[Jakio815]

Minor Logic Change

There was a bug in the read_secure_message() API where SST_SECURE_COMM messages were not being fully read from the socket.

After reading the message header, the implementation is supposed to read the exact number of bytes indicated by that header. However, the previous code called only a single sst_read_from_socket() call, which retrieves only whatever data happens to be buffered at the TCP layer. Since TCP does not guarantee that an entire payload will arrive in one read, the function often returned incomplete message bodies when data is flooding.

This PR introduces a simple loop that continues reading from the socket until the full number of bytes specified in the header has been received.

README updates.

As mentioned in #55, bullet 1 and 2, I updated the README files to be more descriptive.
I created a README under the examples/, with a high level explanation of all examples, and the main README points this file.

Directory name updates

Update scenario_example to SST_Testbed as mentioned in #55. However, didn't change the structure yet.

[sonarqubecloud]

Quality Gate failed

Failed conditions
D Reliability Rating on New Code (required ≥ A)

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[hokeun]

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