feat: improve OpenAI model APIs and examples to better support audio, images, and tool calling

[mattjohnsonpint]

Description

This PR makes several improvements to the OpenAI model interface APIs, for both AssemblyScript and Go.

• Ensures tool calling works in both languages. Previously, we were missing some key properties for the Go SDK.
• Simplifies tool calling (somewhat) with a few well-placed helper functions and convenience methods.
• Adds support to all message types to allow passing content as either strings (as before) or as a list of content parts. In particular, this enables text intput, audio input, or image input for user messages.
• Adds support for audio output as well (for models that support it)
• Adds support for passing either "system" or "developer" messages - which are identical. OpenAI has deprecated "system" messages and prefers "developer" now, but other OpenAI-compatible models may not yet be updated, so both are supported for now.
• Makes a few other minor changes to account for updates to OpenAI's schema
• Adds localtime.isValidTimeZone(tz: string): bool to both SDKs (used by one of the tools in the tool calling examples)

The changes conform to OpenAI's published OpenAPI Schema - which in some areas is more accurate or specific than the end-user documentation.

❗ There are some minor breaking changes with this update, such as the role field of messages now being immutable. It should have little to no impact for most users. Any breaking changes should be noticeable at compile time, and should be resolvable with minimal effort.

❗ This PR is blocked and will stay in draft mode until AssemblyScript/assemblyscript#2894 is merged and released. It will then be updated to include a bump to AssemblyScript in the SDK.

Additionally, the "text generation" examples in both SDKs have been updated as follows:

• Examples showing structured outputs (ie, product generation) have been moved to a new file.
• New examples for media input/output have been added to another new file.
• A new example for tool calling has been added to yet another new file.

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