TLS 1.3 client: Add support for early data writing

[ronald-cron-arm]

This issue tracks the addition of the support for writing early data on client side.

What to implement depends on the result #4902 and thus cannot be fully defined yet.
Things we already know they have to be done:

• Compute the early data transform and switch to it for outbound data after sending the ClientHello

Testing:
The MbedTLS client sends early data as part of a session resumption to an MbedTLS server. The MbedTLS server receives the early data. The handshake may fail if the server is not yet capable of handling early data.

Depends on: #4902, #6329 and #6339

[yuhaoth]

#6486 (comment) is discussing the command line change. It should be think about here.

Sure, here will think about the early data from application.