WIP: sketch out what a services plugin API might look like

[jhkennedy]

Description

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"Ready for review" checklist

• Place this Pull Request (PR) in draft until it is ready for review (see below)
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Merge checklist

• PR title is descriptive
• PR body contains links to related and resolved issues (e.g. closes #1)
• If needed, CHANGELOG.md updated
• If needed, docs and/or README.md updated
• If needed, unit tests added (unsure how? see below!)
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Need help? We welcome contributions at every experience level. You don't have to
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for you, via Continuous Integration (CI),
instead of running them locally. If anything fails and you're not sure why, just
mention @earthaccess-dev/maintainers in a comment and we'll work with you!

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📚 Documentation preview 📚: https://earthaccess--1339.org.readthedocs.build/en/1339/

[github-actions]

👈 Launch a binder notebook on this branch for commit fcbde26

I will automatically update this comment whenever this PR is modified