Add API to generate local host session ticket keys

[raycoll]

The goal of the API(s2n_config_generate_local_ticket_crypto_key) is to make it simpler for applications that want to generate a session ticket key that will be used locally on the host(not shared among a fleet of servers). The API takes no secrets as argument from the user to prevent any possibility of misuse.

Before this API was added, applications generated a session local session ticket by generating secret key material and name themselves and then providing them as arguments to s2n_config_add_ticket_crypto_key.

TODO

• [x] Determine if it still makes sense for s2n_config_generate_local_ticket_crypto_key to take intro_time_in_seconds_from_epoch as argument(should s2n just assume the key should be put in service immediately?
• [x] Update s2nd to use s2n_config_generate_local_ticket_crypto_key by default
• [x] Add unit tests
• [x] Update USAGE-GUIDE.md

Testing:

TODO: unit test.

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