Support secure connection with custom certificates

[larryzhao]

• Add custom_certificates in the initialization options of Etcdv3
• Pass down custom_certificates down to Connection to initialize Channel
• Noway to test the instance of GRPC::Core::ChannlCertificates since the certs variables are locked C layer.

[codecov]

Codecov Report

Merging #125 into master will decrease coverage by 0.05%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   99.08%   99.02%   -0.06%     
==========================================
  Files          30       30              
  Lines        1639     1642       +3     
==========================================
+ Hits         1624     1626       +2     
- Misses         15       16       +1

Impacted Files	Coverage Δ
lib/etcdv3/connection_wrapper.rb	93.93% <100%> (+0.18%)	⬆️
spec/etcdv3/connection_wrapper_spec.rb	100% <100%> (ø)	⬆️
lib/etcdv3.rb	100% <100%> (ø)	⬆️
lib/etcdv3/connection.rb	87.5% <60%> (-3.81%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3a542e8...f4387ad. Read the comment docs.

[codecov]

Codecov Report

Merging #125 into master will decrease coverage by 0.05%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
- Coverage   98.45%   98.39%   -0.06%     
==========================================
  Files          29       29              
  Lines        1614     1617       +3     
==========================================
+ Hits         1589     1591       +2     
- Misses         25       26       +1

Impacted Files	Coverage Δ
lib/etcdv3/connection_wrapper.rb	93.93% <100%> (+0.18%)	⬆️
spec/etcdv3/connection_wrapper_spec.rb	100% <100%> (ø)	⬆️
lib/etcdv3.rb	98.98% <100%> (+0.01%)	⬆️
lib/etcdv3/connection.rb	87.5% <60%> (-3.81%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bbb2e33...b3e28b4. Read the comment docs.

[davissp14]

@larryzhao Hey there, are you still working on this or is this ready for a review?

[larryzhao]

@davissp14 yes, I think it's ok for a review. I am now running it on my production.

Sorry for the codecov. Actually I am a little bit confused about this coverage check, I tried to fix it up but just don't understand where I should fix to make it up to 100% .

[davissp14]

@larryzhao Good stuff. I'll do my best to take a look soon.