ci: add NuGet package signing and verification #85

Job	Run time
build (Release)	36s
build (Debug)	32s
test:core	25s
examples:core / BillingQuotas	26s
test:governance-redis	27s
dependency health	29s
test:governance	27s
examples:governance / RedisQueries	23s
examples:core / IamRoles	18s
examples:core / WorkflowApprovals	23s
examples:core / FeatureFlags	25s
examples:governance / RequestLifecycle	22s
examples:governance / VersionedResolution	31s
examples:governance / DecisionTaxonomy	23s
examples:governance / Queries	27s
examples:governance / GovernedExecution	29s
examples:governance / ApprovalWorkflow	22s
validation summary	2s
	7m 27s

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