Implement memory-based overloads on SftpFileStream

[drieseng]

We should implement the following overloads on SftpFileStream:

• ReadAsync(Memory buffer, System.Threading.CancellationToken cancellationToken = default)
• WriteAsync(ReadOnlyMemory buffer, System.Threading.CancellationToken cancellationToken = default)

When this is done, remove the CA1844 suppression on SftpFileStream.

[v-vpavliuk-eis]

Yep. I ran into such issue.