[minor] support mTLS request to ZTS

[WindzCUHK]

Description

Support requesting ZTS with client certificate.

Changes

1. add enable flag for each sidecar API endpoints
   1. endpoints
      1. ntoken
      2. role token
      3. proxy
   2. hard-coded to true for backward compatibility
      1. inside usecase.New()
      2. remove in next major release
2. when both client certificate and ntoken are set,
   1. use client certificate (endpoint level) over ntoken (application level)
3. disable ntokend
   1. when /ntoken is disabled and NO other endpoints uses ntoken

Type of change

• [ ] Bug fix
• [x] New feature
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Non-code changes (update documentation, pipeline, etc.)

Flags

• [ ] breaks backward compatibility
• [x] requires a documentation update
• [ ] has untestable code

Related issue/PR

Delete if not applicable

• Fixes #38
• Fixes #45

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Checklist

• [x] Followed the guidelines in the CONTRIBUTING document
• [x] Added prefix [major]/[minor]/[patch]/[skip] in the PR title
• [ ] Tested and linted the code
• [ ] Commented the code
• [ ] Made corresponding changes to the documentation
• [ ] Confirmed no dropping in test coverage (by Codecov)
• [ ] Passed all pipeline checking
• [ ] Approved by >1 reviewer

Checklist for maintainer

• [ ] Use Squash and merge
• [ ] Double-confirm the merge message has prefix [major]/[minor]/[patch]/[skip]
• [ ] Delete the branch after merge

[codecov-io]

Codecov Report

Merging #44 (2f0783a) into master (1e6172a) will increase coverage by 1.12%. The diff coverage is 98.50%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   90.86%   91.98%   +1.12%     
==========================================
  Files          13       13              
  Lines        1127     1136       +9     
==========================================
+ Hits         1024     1045      +21     
+ Misses         79       71       -8     
+ Partials       24       20       -4     

Impacted Files	Coverage Δ
usecase/tenantd.go	94.68% <93.33%> (+11.13%)	:arrow_up:
config/config.go	100.00% <100.00%> (+11.76%)	:arrow_up:
main.go	53.48% <100.00%> (-0.36%)	:arrow_down:
router/routes.go	100.00% <100.00%> (ø)
service/access.go	94.14% <100.00%> (+0.22%)	:arrow_up:
service/role.go	94.63% <100.00%> (+0.21%)	:arrow_up:
service/server.go	94.91% <100.00%> (-0.40%)	:arrow_down:
service/svccert.go	92.43% <100.00%> (-0.43%)	:arrow_down:
service/tls.go	96.22% <100.00%> (+1.48%)	:arrow_up:
... and 11 more

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