refactor: consolidate platform-credentials on token-exchange/store

[robertodauria]

Summary

• Use token-exchange/store.AutojoinManager in main service (fixes namespace mismatch: was "autojoin", now "platform-credentials")
• Remove duplicated internal/adminx/datastore.go (now uses token-exchange/store)
• Remove dead code: internal/adminx/apikeys.go (unused GCP API Keys system)
• Remove dead code: internal/adminx/keysiface/keys.go

Context

The platform-credentials system evolved organically, creating inconsistencies:

• orgadm was already using token-exchange/store with namespace "platform-credentials"
• The main autojoin service was using a local internal/adminx/datastore.go with namespace "autojoin"

This PR consolidates both to use token-exchange/store with the correct "platform-credentials" namespace.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 21392512518

Details

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.6%) to 72.177%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
main.go	0	10	0.0%

Files with Coverage Reduction	New Missed Lines	%
internal/adminx/namer.go	9	70.0%

Totals
Change from base Build 21389054675:	1.6%
Covered Lines:	1253
Relevant Lines:	1736

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💛 - Coveralls

[bassosimone]

Please, make sure the datastore entities in autojoin and the entities in platform-credentials are equal before merging this pull request 🙏.

Motivation for the ask: in this PR and in #88, we are basically moving m-lab/autojoin away from the autojoin namespace.

Otherwise, trying to consolidate code that looks similar and should probably not be separated seems good to me. On this note, please, make sure that the commit you merge contains context about the overall plan that we're executing here (we're not using issues and commits are part of the repository history much more integrally than issues anyway, so it's probably good to have good documentation of the why).

[robertodauria]

All the datastore entites in autojoin have been migrated to platform-credentials.

I have added another small but important piece: the ability to read credentials from adifferent project (-credentials-project). This way, we can really consolidate: the autojoin service verifies legacy API keys against mlab-oti.platform-credentials. When the JWT flow is used, token-exchange verifies API keys against the same datastore namespace, which becomes the only source of truth.