[playground] llm span to playground parameter parsing utils

[mikeldking]

Analyze the contents of an LLM Span (e.g. the attributes) and parse out as much structured data (e.g. data to be passed to the playground) as possible. This utility should be used to mount state in the playground.

• [playground] accumulate errors while parsing span attributes for prompt playground #4922
• parse output messages
• parse invocation parameters
• add fallbacks for variant input / output
  • input messages with single content
  • input messages with multipart content
  • input.value
  • prompt templates
• parse variables
• parse templates?
• [playground][ui] parse tool calls from span input messages for span replay #5180