Add JWT public signing key auto-refresh

[letmaik]

Implements #1893.

Follow-up PRs (not necessarily all by myself):

• Expose connection failures to enclave to be able to record key refresh errors in those situations
• In case of any error during key refresh, schedule next task with lower time interval, maybe gradually backing off
• Do an initial immediate key refresh after KV is synced (not sure where the best place is to trigger this)
• Move update_ca_cert proposal to C++, currently in Lua. Also, rename to set_ca_cert and add remove_ca_cert proposal.

[ghost]

letmaik/jwt-key-auto-refresh@15658 aka 20201123.18 vs master ewma over 50 builds from 15114 to 15645