feat: support sni and skipverify option for etcd tls

[druggo]

Why submit this pull request?

• [ ] Bugfix
• [x] New feature provided
• [ ] Improve performance
• [ ] Backport patches

What changes will this PR take into?

add two option for etcd tls connection:

• sni: the servername of the etcd server certificate
• skipverify: skip etcd server certificate verify

Related issues

resolve #2808 #2815

Checklist:

• [x] Did you explain what problem does this PR solve? Or what new features have been added?
• [ ] Have you added corresponding test cases?
• [ ] Have you modified the corresponding document?
• [x] Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #2850 (3ddb44f) into master (e8bc9b9) will decrease coverage by 14.55%. The diff coverage is n/a.

:exclamation: Current head 3ddb44f differs from pull request most recent head 5545843. Consider uploading reports for the commit 5545843 to get more accurate results

@@             Coverage Diff             @@
##           master    #2850       +/-   ##
===========================================
- Coverage   70.55%   56.01%   -14.55%     
===========================================
  Files         201      183       -18     
  Lines        7846     7102      -744     
  Branches      881      877        -4     
===========================================
- Hits         5536     3978     -1558     
- Misses       2001     2914      +913     
+ Partials      309      210       -99     

Flag	Coverage Δ
backend-e2e-test-ginkgo	?
backend-unit-test	49.95% <ø> (ø)
frontend-e2e-test	61.69% <ø> (-7.39%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
api/internal/conf/conf.go	66.22% <ø> (ø)

... and 68 files with indirect coverage changes

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