[EPIC] MultiModal Refactor

[logan-markewich]

We need to improve multi-modal support in the framework.

To do this, we start from the basic primitives (chat messages, prompt templates, llms) and then expand into better support across retrievers, response synthesizers, indexes

• Expand chat messages to have dedicated content types for multi-modal #15948
• Consolidate MultiModal LLMs with Base LLMs #15949
• Merge all the schema functionalities into Node and Document, schedule everything else to be removed in 1.0