Guess image_mimetype for ImageNode

[nerdai]

Description

This PR introduces a couple of enhancements when working with ImageNode's:

1. It guesses the image_mimetype field from image_path if provided. (This is somewhat breaking as before we were guessing the mimetype from the path)
2. Similar to TextNode we add forward-compatibility by enabling construction via a image media resource type.

Also, I made an implementation change for validating data and mimetype fields of a MediaResource. Specifically, I opt to use field_validator over model_validator to be able to handle the validation of these two separate fields "separately".

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change

[logan-markewich]

Looks great! Thanks @nerdai 🫡