feat: update date generator to support backfills

[ceholden]

What I am changing

This PR updates the functionality of the "date generator" Lambda function to support backfills. Currently the function will always run relative to "now", which is great for forward processing but means we can't invoke this as if we were in the past. Specifically I am making this change to catch ~half of a day of granules from Sentinel-2C on January 21, 2025 that we did not catch in the 5 day lookback we have with our forward processing.

How I did it

I took advantage of the changes @chuckwondo made to facilitate testing the "date generator" without needing to do things like patch datetime for time travel. I extended the changes he made to allow these inputs (platforms, "now", and lookback days) to come from the Lambda function payload rather than through kwargs used by unit tests.

This allows us to invoke the StepFunction with a payload like,

{
    "now": "2025-01-22",
    "lookback_days": 2,
    "platforms": ["S2B", "S2C"],
    // rest of StepFunction default input payload
    ...,
}

How you can test it

I updated the unit tests to pass the inputs via event payload rather than extra kwargs,

cd lambdas/date_generator
make test

[ceholden]

Aah our integration tests are failing because the "mock Scihub" components don't support S2C...

[ERROR] FileNotFoundError: [Errno 2] No such file or directory: '/var/task/scihub_responses/scihub_response_platform_S2C_index_1_yesterday.json'
Traceback (most recent call last):
  File "/var/task/handler.py", line 24, in handler
    body = (responses_dir / response_fixture).read_text()
  File "/var/lang/lib/python3.11/pathlib.py", line 1058, in read_text
    with self.open(mode='r', encoding=encoding, errors=errors) as f:
  File "/var/lang/lib/python3.11/pathlib.py", line 1044, in open
    return io.open(self, mode, buffering, encoding, errors, newline)

Fixing...

[ceholden]

Rather than fiddling with the mocked responses I figured we can just request platforms that we know are supported by the mocked API. This will also help us avoid an error when ESA launches Sentinel-2D in >=2028 😄

Probably more importantly this also helps us poke to make sure our StepFunction can handle overriding the "platform" to the "date generator"

[chuckwondo]

Great! Nice to see this could be implemented with so few code changes.

Approving, so feel free to ignore my only comment about type annotation.

[chuckwondo]

Sweet!