Add client side TLS cert support

[jialindai]

Enable configuring config client side TLS certificate and trust store.

[codecov]

Codecov Report

Merging #1679 into 2.2.x will decrease coverage by 0.21%.
The diff coverage is 47.82%.

@@             Coverage Diff              @@
##              2.2.x    #1679      +/-   ##
============================================
- Coverage     79.84%   79.63%   -0.22%     
- Complexity     1208     1211       +3     
============================================
  Files           156      156              
  Lines          4521     4541      +20     
  Branches        607      606       -1     
============================================
+ Hits           3610     3616       +6     
- Misses          686      698      +12     
- Partials        225      227       +2     

Impacted Files	Coverage Δ	Complexity Δ
...fig/client/ConfigServicePropertySourceLocator.java	72.28% <29.41%> (-4.78%)	32.00 <1.00> (+1.00)	⬇️
...rk/cloud/config/client/ConfigClientProperties.java	92.06% <100.00%> (+0.39%)	46.00 <3.00> (+3.00)
...erver/environment/NativeEnvironmentRepository.java	85.51% <0.00%> (-1.38%)	45.00% <0.00%> (-1.00%)

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[jialindai]

@spencergibb , could you help to review the PR?

[jialindai]

@spencergibb , this PR has been updated to use common classes in spring-cloud/spring-cloud-commons#803

Could you help to review?

[spencergibb]

@ryanjbaxter move to test module

[spencergibb]

@ryanjbaxter remove and use one from commons