Fix s1_orbit to use ASF S3 orbit source

[scottstanie]

Scihub has been broken, leading to broken functionality for s1_orbit1. This is also breaking local COMPASS use due to opera-adt/COMPASS#226

You should be able to run (random SAFE name):

import s1reader.s1_orbit

orbit_path =  s1reader.s1_orbit.get_orbit_file_from_dir("S1A_IW_SLC__1SDV_20240902T233705_20240902T233732_055493_06C53E_4976", ".", auto_download=True)

and get S1A_OPER_AUX_POEORB_OPOD_20240922T070612_V20240901T225942_20240903T005942.EOF

This takes the idea from scottstanie/sentineleof#66 to use the new ASF public orbit source.

Removed:
• The global Scihub credentials
• Functions that queried the Copernicus GNSS API (check_internet_connection, _get_orbit_dict, _download_orbit_file)

Added/Updated:

• We no longer need to have an existing .zip file to run this (I'm not sure why we had the exist check: we just parse the name for start/stop dates, so we don't need an actual file.
• A new S3-based listing function (list_public_bucket()) and orbit file getter (get_orbit_files())
• A helper to download orbit files from ASF S3 (download_orbit_file_from_s3())
• An updated retrieve_orbit_file() that works with the S3 service, including fallback to handling pair concatenation if needed
  This should “fix” the broken module by removing the dead Scihub parts while still providing the previous functionality using the new ASF S3 service.

I've tried to keep as much old stuff as possible, but I'm not 100% clear which portions are necessary for current OPERA production and which are handled by the PCM staging logic. I'm most unsure around the concatenation logic.

Closes #3