New endpoints for Assistants

[joacir]

Include support to handle the new Assistants API #115 :

Assistants (beta)

• Create assistant
• Retrieve assistant
• Modify assistant
• Delete assistant
• Lists assistants
• Create assistant file
• Retrieve assistant file
• Delete assistant file
• List assistant files

Threads (beta)

• Create thread
• Retrieve thread
• Modify thread
• Delete thread

Messages (beta)

• Create message
• Retrieve message
• Modify message
• Lists messages
• Retrieve message file
• List message files

Runs (beta)

• Create run
• Retrieve run
• Modify run
• Lists runs
• Submit tool outputs
• Cancel run
• Create thread and run
• Retrieve run step
• List run steps

[orhanerday]

LGTM!
Thank you so much @joacir!