ApiView: port smaller set of feedback changes

[trrwilson]

A few of the changes in #42 were contentious and we'd like to extract some of the more straightforward updates for the March release target.

This pulls in several of the aforementioned PR's changes:

• OpenAIClientOptions now appropriate derives from PipelineClientOptions
• GetLegacyCompletionClient is removed from OpenAIClient
• Name properties have been clarified in non-trivial cases:
  • Several Name properties pertaining to functions are renamed to FunctionName
  • Message-specific participant Name properties are renamed to ParticipantName
• Public setters on request models are removed, replaced where appropriate with init
• Response model collection types are now initialized to empty where they were previously null
• ChatMessageContent type conversions to string and (new) Uri are explicit instead of implicit
• CreatedAt is now present on GeneratedImageCollection (in addition to its existing copy-placement in constituent GeneratedImage instances)
• (new) Assistants override properties are renamed to e.g. InstructionsOverride to avoid misleading apparent verb symbols like OverrideInstructions

Incidentals:

• Many files have accumulated using statements cleaned up
• batch and batch_output are added to OpenAIFilePurpose
• pcm is added to transcription/translation formats

Intentionally deferred:

• Introduction of nullable reference types (contentious, complex)
• Comprehensive serialization update w/helper (complex)
• Rename of content to text on ChatRequestAssistantMessage (contentious)