Title: Pydantic versioning

Description:

• Handled case of user's pydantic v1 dependency
• Added types for beta.chat.completion
• Added type for chat.completion
• Added extra="allow" for all the types
• Setting None as default value for the Optional Keys
• Adding more type classes for the Threads route
• Minimum required pydantic version 1.10.8 (advised to use 2.0 [suggested, as per pydantic docs])

Motivation:
User reported issue, pydantic version mismatch while using other SDKs

Related Issues:
Closes: #247 #252