Dump current tenant config

[SomeoneToIgnore]

The PR adds an endpoint to show tenant's current config: GET /v1/tenant/:tenant_id/config

Tenant's config consists of two parts: tenant overrides (could be changed via other management API requests) and the default part, substituting all missing overrides (constant, hardcoded in pageserver).
The API returns the custom overrides and the final tenant config, after applying all the defaults.

Along the way, it had to fix two things in the config:

• allow to shorten the json version and omit all null's (same as toml serializer behaves by default), and to understand such shortened format when deserialized. A unit test is added
• fix a bug, when PUT /v1/tenant/config endpoint rewritten the local file with what had came in the request, but updating (not rewriting the old values) the in-memory state instead.
  That got uncovered during adjusting the e2e test and fixed to do the replacement everywhere, otherwise there's no way to revert existing overrides. Fixes Changing tenant configuration writes only changed values to disk, continues with different config #3471 (commit dc688af)
• fixes Tenant configuration is modified even if file saving fails #3472 by reordering the config saving operations

[koivunej]

The second bullet point sounds like this will fix #3471. Could you confirm, edit Fixes #3471 into the description?

[SomeoneToIgnore]

Yes, looks it is the one. I've embedded the test for this fix with the new http endpoint value checks, but presumably can use old sql api to check these values too and separate into another PR.
Not sure if it's worth it, though.

The fix without tests is in its own commit: dc688af

[koivunej]

I think this is looking good. As always, some related and unrelated questions :)

[koivunej]

Oh yeah, we should update openapi.

[SomeoneToIgnore]

we should update openapi

This time, I agree fully.
Interesting, that the API schema is simple and reuses the response schema which exactly what I'd like to see in the rust code.

[problame]

This mostly LGTM, apart from the discussion regarding persistence of the tenant config.

Let's unblock this PR by changing it to the old behavior (apply in memory, and best-effort save to disk).

[koivunej]

Aah yes, lets unblock this. Please include the other fixed issue in the description as well! Both #3471 and #3472. Well, at least mention it if the old behaviour on disk-full is preserved.

[SomeoneToIgnore]

Thank you, now have understood indeed and added another issue in the description.
Looks like the current non-updateable behavior is fine for now, so merging the PR.