[ci] Expire governance lock substitutions

Fixes #164.

Mirrors #176 for substitutions:

• Ingest GovernanceLockSubstitution in SvDsoStore and list proposals past expiresAt.
• ExpireGovernanceLockSubstitutionTrigger exercises DsoRules_GovernanceLockSubstitution_DsoExpire.
• The query is hand-written because the parties to skip are the proposer at owner and the lock owner at nested target.owner, which ignoredPartyFields cannot reach.
• foldSpecification now takes a GovernanceLockSpecification rather than a GovernanceLock, so substitutions can resolve observers from target.specification. Added governanceLockObservers, mirroring the Daml function of the same name.

Draft: the integration test is still to come, pending #181 restructuring GovernanceLockTimeBasedIntegrationTest.