Update Vectara Index to return both document level and page level metadata

[ofermend]

Description

Added Vectara part_metadata to returned Node metadata.
Now the "metadata" in each node is the part_metadata from Vectara, and document level metadata comes as a sub-dictionary with the key "document".

For example:
Node(
...
metadata = {
'document': {'url': 'https://vectara.com/some-page' }
'page': 5
}
)

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Breaking change (For applications that depend on the old structure metadata value)

How Has This Been Tested?

• I added new unit tests to cover this change

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods