Re-write proxy server to use per-revision config

[eseidel]

We could delete the central config for the proxy server and instead make it per-revision config.

This would just mean we upload download.shorebird.dev/$revision/engine.version or similar that gives us an id that we're mapping back to.

The proxy server would then cache these mappings in memory.

[eseidel]

Proposal:

// Keeps a (fixed size) LRU cache of recently requested shorebird versions.
// First time it sees a new engine version it makes a request to storage
// for $shorebird/manifest.yaml.
// If that fails, it records and entry in the cache noting that's an invalid
// version.
// If it succeeds it records the version in the cache and returns it.

manifest.yaml:

shorebird_engine_revision: e6a2a5a43973430d9f038cd81cb1779b6b404909
flutter_engine_revision: ec975089acb540fc60752606a3d3ba809dd1528b
artifact_overrides:
  # artifacts.zip
  - flutter_infra_release/flutter/$engine/android-arm-64-release/artifacts.zip
  - flutter_infra_release/flutter/$engine/android-arm-release/artifacts.zip
  - flutter_infra_release/flutter/$engine/android-x64-release/artifacts.zip
  # embedding release
  - download.flutter.io/io/flutter/flutter_embedding_release/1.0.0-$engine/flutter_embedding_release-1.0.0-$engine.pom
  - download.flutter.io/io/flutter/flutter_embedding_release/1.0.0-$engine/flutter_embedding_release-1.0.0-$engine.jar
  # arm64_v8a release
  - download.flutter.io/io/flutter/arm64_v8a_release/1.0.0-$engine/arm64_v8a_release-1.0.0-$engine.pom
  - download.flutter.io/io/flutter/arm64_v8a_release/1.0.0-$engine/arm64_v8a_release-1.0.0-$engine.jar
  # armeabi_v7a release
  - download.flutter.io/io/flutter/armeabi_v7a_release/1.0.0-$engine/armeabi_v7a_release-1.0.0-$engine.pom
  - download.flutter.io/io/flutter/armeabi_v7a_release/1.0.0-$engine/armeabi_v7a_release-1.0.0-$engine.jar
  # x86_64 release
  - download.flutter.io/io/flutter/x86_64_release/1.0.0-$engine/x86_64_release-1.0.0-$engine.pom
  - download.flutter.io/io/flutter/x86_64_release/1.0.0-$engine/x86_64_release-1.0.0-$engine.jar

Thoughts?

[eseidel]

One problem with this plan is that we need to know what the urls look like to be able to find ones with engine versions in them. So we'll need some regexps. :(

[eseidel]

Which again gets back to we may need to know what all urls from flutter_tools look like. If we did we could then 404 on urls we don't expect which might help catching missed redirects later (or underlying tool changes causing it to not fetch the urls we expect it to).