tarantool-etcd-hco iproto-impl: etcd_election_observe vs Election.Observe Lined Up

Wrap the existing handler election.observe (already invoked by gRPC/JSON via defs.lua route v3electionpb.Election.Observe) as a Lua global etcd_election_observe callable via iproto CALL. Mechanical change in app/etcd/api.lua: add rawset(_G, 'etcd_election_observe', wrap('election', 'observe')) under the appropriate service block; append 'etcd_election_observe' to the funcs table so box.schema.func.create plus guest grant runs at role apply; document the wrapper in the module-level LDoc comment. If Election.Observe returns UNIMPLEMENTED today, the wrapper still gets added so the iproto caller observes the same UNIMPLEMENTED error gRPC would return. Election.Observe is server-streaming over gRPC; iproto has no stream primitive, so propose a polling shape (observe_create/_pull/_cancel) mirroring the Watch precedent at api.lua:81-83.

etcd_election_observe is callable from net.box:call('etcd_election_observe', {req}) with no Lua error before handler dispatch. Successful request shape produces same successful response shape as gRPC. box.schema.func.create plus guest grant idempotent on role re-apply.

Epic tarantool-etcd-w8q. Handler at app/etcd/election.lua.