add context path support and add helm sample

[nobodyiam]

What's the purpose of this PR

1. add context path support
2. add user defined config files and spring profiles support
3. add helm sample

Which issue(s) this PR fixes:

Fixes #3201

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[codecov-commenter]

Codecov Report

Merging #3205 into master will decrease coverage by 0.04%.
The diff coverage is 60.00%.

@@             Coverage Diff              @@
##             master    #3205      +/-   ##
============================================
- Coverage     51.27%   51.22%   -0.05%     
+ Complexity     2274     2272       -2     
============================================
  Files           439      439              
  Lines         13622    13626       +4     
  Branches       1385     1385              
============================================
- Hits           6984     6980       -4     
- Misses         6151     6157       +6     
- Partials        487      489       +2     

Impacted Files	Coverage Δ	Complexity Δ
...apollo/portal/controller/PrefixPathController.java	50.00% <60.00%> (ø)	1.00 <1.00> (ø)
...rk/apollo/spring/property/SpringValueRegistry.java	83.33% <0.00%> (-5.56%)	10.00% <0.00%> (-1.00%)
...ework/apollo/internals/RemoteConfigRepository.java	86.25% <0.00%> (-3.13%)	24.00% <0.00%> (-1.00%)
.../framework/apollo/spring/property/SpringValue.java	87.71% <0.00%> (-1.76%)	20.00% <0.00%> (-1.00%)
.../apollo/internals/RemoteConfigLongPollService.java	78.31% <0.00%> (+1.20%)	29.00% <0.00%> (+1.00%)

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