docs: add configurable pipeline design proposal

[Abhishek9639]

Description of the PR

This PR adds a design proposal document for making the GUAC Collector-Certifier-Queryer pipeline configurable, as discussed in issue #1093.

Right now, running GUAC in production means spinning up a separate binary for every collector and certifier guaccollect osv, guaccollect deps_dev, guaccollect scorecard, etc. That gets operationally heavy pretty quickly, especially for teams just getting started. On top of that, there's no way to look at guac.yaml and understand what components are actually running the flat config doesn't map to individual components at all.

The document in proposals/configurable-pipeline.md captures both of these problems clearly and sketches out a proposal: a pipeline: block in guac.yaml where users can declare which collectors and certifiers are enabled, so eventually a single guacone run command could bring up the whole pipeline from one config file.

This is intentionally left as a work-in-progress proposal to gather feedback from the community before any implementation starts.

Fixes #1093

PR Checklist

• All commits have a Developer Certificate of Origin (DCO) -- they are generated using -s flag to git commit.
• All new changes are covered by tests
• If GraphQL schema is changed, make generate has been run
• If GraphQL schema is changed, GraphQL client updates/additions have been made
• If OpenAPI spec is changed, make generate has been run
• If ent schema is changed, make generate has been run
• If collectsub protobuf has been changed, make proto has been run
• All CI checks are passing (tests and formatting)
• All dependent PRs have already been merged

[Abhishek9639]

Hey @jeffmendoza or @mlieberman85,
I've added a design proposal doc at proposals/configurable-pipeline.md capturing the two problems and a rough sketch of the pipeline: config idea. Kept it WIP intentionally would love your feedback before moving toward implementation.
Thanks!

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