add KpMaintence proto

[insipx]

Add KpMaintenance message type to MLS database task proto

Adds a new empty KpMaintenance message to task.proto, representing a durable singleton task for key-package maintenance. The Task message gains a new kp_maintenance oneof variant (field 4), allowing tasks of this type to be encoded and decoded alongside existing variants.

^{Macroscope summarized 503dc57.}

[macroscopeapp]

Approvability

Verdict: Approved

Additive proto schema change that adds a new empty message type and corresponding field. The change is backwards-compatible and the author is the designated code owner of the modified file.

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