Add experimental "subgraph cache" plugin

[SimonSapin]

Add a new "subgraph cache" plugin that starts as a copy of the "entity cache" plugin, but:

• With a more accurate name
• Marked (for now) experimental instead of preview
• Hidden from schema config (until it’s ready for use)
• Intended to change significantly (in later PRs) while "entity cache" stays unchanged, to avoid disrupting existing users and decouple the eventual migration from Router version upgrades

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Metrics and logs are added³ and documented
• Tests added and passing⁴
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. A lot of (if not most) features benefit from built-in observability and debug-level logs. Please read this guidance on metrics best-practices. ↩

4. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[github-actions]

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[SimonSapin]

I’ve split the commits to try and make review easier, so I suggest reviewing by commit.

At the moment the configuration::tests::schema_generation test fails because 7117b24 effectively broke HIDDEN_FROM_CONFIG_JSON_SCHEMA. I don’t know what pattern_properties is supposed to do, but stuff intended to be hidden ends up being visible in the config schema. @rregitsky, what was the motivation for that commit?