contrib: add gRPC gateway script for user/add with full chain TLS.

[hexfusion]

Show automated authentication workflow example in bash using TLS and curl.

ref: #9691

[codecov-io]

Codecov Report

Merging #9692 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9692      +/-   ##
==========================================
- Coverage   68.91%   68.76%   -0.15%     
==========================================
  Files         373      373              
  Lines       34829    34829              
==========================================
- Hits        24001    23949      -52     
- Misses       9110     9155      +45     
- Partials     1718     1725       +7

Impacted Files	Coverage Δ
etcdserver/api/v3rpc/lease.go	65.11% <0%> (-10.47%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-5.74%)	⬇️
pkg/adt/interval_tree.go	88.58% <0%> (-3.31%)	⬇️
lease/leasehttp/http.go	56.61% <0%> (-2.95%)	⬇️
clientv3/concurrency/election.go	79.52% <0%> (-2.37%)	⬇️
etcdserver/api/v3rpc/watch.go	74.71% <0%> (-1.92%)	⬇️
raft/storage.go	87.96% <0%> (-1.86%)	⬇️
proxy/grpcproxy/lease.go	79.18% <0%> (-1.36%)	⬇️
rafthttp/peer.go	77.65% <0%> (-1.12%)	⬇️
etcdserver/cluster_util.go	58.74% <0%> (-0.9%)	⬇️
... and 14 more

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[gyuho]

lgtm thanks!