[Obs AI Assistant] Rename API endpoint for chat complete and references to function and function_call

[viduni94]

Before publishing the docs for our public chat complete API, we need to make sure that the properties and API endpoint conform with up to date terminology.

For example:

• We have API params named function_call, which needs to be renamed to tool_call
• And all references to function needs to be renamed to tool
• The API endpoint path needs to be renamed as well -- we also need to align with security and unification efforts

Relates to https://github.com/elastic/obs-ai-assistant-team/issues/193
Draft PR for the public API documentation - #206869

[elasticmachine]

Pinging @elastic/obs-ai-assistant (Team:Obs AI Assistant)

[viduni94]

The API endpoint pathname decision is pending, therefore will move this to Blocked until we have a clear direction
cc: @teknogeek0 @emma-raffenne