add Galileo recipe

[sahiljhawar]

Summary

GSSC Galileo EMU recipe (solves #118 )

Checklist

• SPDX header present on new/changed files, with yourself as SPDX-FileContributor.
• ruff check . / ruff format . and ty check pass (or pre-commit ran clean).
• Tests added/updated as needed; pytest -m basic (and/or -m visual) passes locally.
• README.md / CONTRIBUTING.md updated if this changes install steps, usage, or the processing-script list.
• pre-commit executed without any failures

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New processing recipe / satellite mission

• Mission/instrument:
• Data product(s):
• Related issue:

Checklist

• Recipe lives under el_paso/recipes/<mission>/ and follows the structure of an existing recipe (input handling, metadata, output to PRBEM-style standard names).
• el_paso/recipes/<mission>/__init__.py updated if a new module was added.
• README.md "Available processing scripts" list updated to include this recipe.
• Type hints added on new/changed code.
• A test covers the new recipe.
• If the recipe needs new reference/system test data, it's been added to the Zenodo dataset (or flagged in this PR for a maintainer to do so).
• If the recipe needs credentials/secrets to download data (e.g. ESA_CLIENT_ID), that's called out below.

Recipes needs GSSC_USER and GSSC_PASS