[RFC] Use pydantic

[gadomski]

Overview

This is a total rewrite of the library to use pydantic under the hood for validation and input/output schema generation. Key points:

• All tasks are now pydantic models, and their input and output data structures are also pydantic models. This allows jsonschema generation for input, output, and task config.
• Custom subclasses of Task are provided for common use-cases

class definition	key method
Task	process(self, input: List[Any]) -> List[Any]
StacOutTask	process_to_items(self, input: List[Any]) -> List[Item]
StacInStacOutTask	process_items(self, input: List[Item]) -> List[Item]
OneToManyTask	process_one_to_many(self, input: Any) -> List[Any]
OneToOneTask	process_one_to_one(self, input: Any) -> Any
ToItemTask	process_to_item(self, input: Any) -> Item
ItemTask	process_item(self, item: Item) -> Item
HrefTask	process_href(self, href: str) -> Item

• Models are also included for Payload and its substructures. This will hopefully solidify the data model for payloads (there had been confusion in the past around dict-or-list for process, etc)
• Handling has been moved up to Payload, so tasks just worry about themselves
• I'm using stac-asset instead of fsspec for read (and eventually, write)
• An example is provided of how to make a simple item using rio-stac, both as a standalone file and as a package. See the examples/ directory for more information.
• CLI provides stac-task list for showing available tasks, and stac-task jsonschema TASK MODEL for dumping the jsonschema
• I've changed the package name to stac_task (from stactask) -- I figure this is a good chance to bring the naming in line with other repos AND clearly signal these very breaking changes to downstreams

Opening as a draft PR for visibility and feedback.

Task list

• Flesh out documentation
• Unit test every generic class
• Get the plugin story sorted out
• Provide more examples of building custom tasks and how they would be called / invoked
• Asset download/upload helpers
  • As an aside, I'm not sure how much of this we actually want to put in stac-task, and how much should be farmed out to stac-asset
• Provide some common "helper" tasks, such as a pystac-client search. Maybe even include rio-stac via an extra dependencies set.
• Update Github branch protection for the new CI job names
• Build and deploy documentation with read the docs

[gadomski]

Closing as OBE. Maybe we'll re-visit someday.