functional-tester: enable TLS, phase 1

[gyuho]

Locally ran test suites, and confirmed that it works.

Use etcd --auto-tls --peer-auto-tls flag, and send client requests with clientv3.Config.InsecureSkipVerify: true. Next, generate static TLS assets with no auto TLS flags, like we do in production. And cache TLS assets from previous runs.

Fix #8943.

[codecov-io]

Codecov Report

Merging #9534 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9534      +/-   ##
==========================================
- Coverage   72.39%   72.39%   -0.01%     
==========================================
  Files         367      367              
  Lines       31267    31267              
==========================================
- Hits        22636    22635       -1     
- Misses       6996     7000       +4     
+ Partials     1635     1632       -3

Impacted Files	Coverage Δ
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
mvcc/watcher.go	96.29% <0%> (-3.71%)	⬇️
proxy/httpproxy/director.go	80% <0%> (-2.86%)	⬇️
etcdserver/api/v3election/election.go	66.66% <0%> (-2.78%)	⬇️
rafthttp/msgappv2_codec.go	69.56% <0%> (-1.74%)	⬇️
clientv3/watch.go	94.85% <0%> (-0.99%)	⬇️
proxy/grpcproxy/lease.go	78.28% <0%> (-0.91%)	⬇️
pkg/proxy/server.go	58.81% <0%> (-0.34%)	⬇️
... and 11 more

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