chore: align activate-key with D-R

[paullatzelsperger]

What this PR changes/adds

Aligns the "active-key" operation with the decision record

Why it does that

Briefly state why the change was necessary.

Further notes

• adds the KeyPairActivated event (and associated methods), which is consumed by the DidDocumentService
• adds a keyContext field to the KeyPairResource (and the associated DB schema) to track the "context" of the key, e.g. "JsonWebKey2020"
• renames KeyPairStates#ACTIVE to KeyPairStates#ACTIVATED for consistency
• the PArticipantContextEventCoordinator emits a warning log if a participant is added and activated, but its only key is in the "inactive" state
• KeyPairService: when a key is added in the active state, the activateKey method is invoked in the same transaction
• DidDocumentService reacts to the KeyPairActivated event instead of KeyPairAdded

Linked Issue(s)

Closes # <-- insert Issue number if one exists

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