Add PC STAC client configuration

[giorgiobasile]

Provides a PC-configured PySTAC client, able to sign assets, even those requiring a subscription key.

See PC - Using Tokens for Data Access tutorial for more information regarding SAS Tokens and assets' signing.

Closes #6

Example

from prefect import flow
from prefect_planetary_computer import PlanetaryComputerCredentials

@flow
def example_get_stac_catalog_flow():

    pc_credentials_block = PlanetaryComputerCredentials(
        subscription_key = "sub-key",
        hub_api_token = "hub-token"
    )
    catalog = pc_credentials_block.get_stac_catalog()
    
    # Search STAC catalog for Landsat Collection 2 Level 2 items
    time_range = "2020-12-01/2020-12-31"
    bbox = [-122.2751, 47.5469, -121.9613, 47.7458]
    search = catalog.search(collections=["landsat-c2-l2"], bbox=bbox, datetime=time_range)
    
    items = search.get_all_items()
    return len(items)
    
example_get_stac_catalog_flow()

Screenshots

Checklist

• References any related issue by including "Closes #" or "Closes ".
  • If no issue exists and your change is not a small fix, please create an issue first.
• Includes tests or only affects documentation.
• Passes pre-commit checks.
  • Run pre-commit install && pre-commit run --all locally for formatting and linting.
• Includes screenshots of documentation updates.
  • Run mkdocs serve view documentation locally.
• Summarizes PR's changes in CHANGELOG.md