Adding support for field descriptions

[brimoor]

Resolves #2163

Adds support for storing field-level descriptions on datasets.

This information can be viewed in the App via a new field tooltip (thanks @ritch!) by hovering over the field or attribute names in the App's sidebar! 🥳

Example usage

import fiftyone as fo
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart")
dataset.add_dynamic_sample_fields()

field = dataset.get_field("ground_truth")
field.description = "Ground truth annotations"
field.info = {"url": "https://fiftyone.ai"}
field.save()  # must save after edits

field = dataset.get_field("ground_truth.detections.area")
field.description = "Area of the box, in pixels^2"
field.info = {"url": "https://fiftyone.ai"}
field.save()  # must save after edits

dataset.reload()

field = dataset.get_field("ground_truth")
print(field.description)  # Ground truth annotations
print(field.info)  # {'url': 'https://fiftyone.ai'}

field = dataset.get_field("ground_truth.detections.area")
print(field.description)  # Area of the box, in pixels^2
print(field.info)  # {'url': 'https://fiftyone.ai'}

Field metadata may also be provided when the field is declared, if desired:

import fiftyone as fo

dataset = fo.Dataset()
dataset.add_sample_field(
    "int_field", fo.IntField, description="An integer field"
)

field = dataset.get_field("int_field")
print(field.description)  # An integer field

[ehofesmann]

I love this syntax!

field = dataset.get_field("ground_truth")
field.description = "Ground truth annotations"
field.info = {"url": "https://fiftyone.ai"}
field.save()  # must save after edits

[benjaminpkane]

LGTM