Simplify Element type by use of Pydantic?

[ctrahey]

This can be thought of as a request for a chore rather than an issue.

Pydantic would simplify the first 150 lines of this section of code down to about 50 but more importantly, drop the number of places an author would need to touch when adding/modifying a field from 3 to 1.

There are also improvements to be expected in other use cases, like serialization and other downstream uses that make reference to this type.

unstructured/unstructured/documents/elements.py

Lines 149 to 160 in 27fa2a3

	class ElementMetadata:
	"""Fully-dynamic replacement for dataclass-based ElementMetadata."""
	# NOTE(scanny): To add a field:
	# - Add the field declaration with type here at the top. This makes it a "known" field and
	# enables type-checking and completion.
	# - Add a parameter with default for field in __init__() and assign it in __init__() body.
	# - Add a consolidation strategy for the new field in
	# `ConsolidationStrategy.field_consolidation_strategies()` below. This strategy will be used
	# to consolidate this new metadata field from each pre-chunk element during chunking.
	# - Add field-name to DEBUG_FIELD_NAMES if it shouldn't appear in dict/JSON or participate in
	# equality comparison.