Add composite provider to support multiple network providers

[lujiajing1126]

Ⅰ. Describe what this PR does

Currently, only a single trafficRouting network provider is supported even multiple ones are defined.

To accomplish this, a composite provider is needed. A composite provider is just an array of NetworkProvider.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Special notes for reviews

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[codecov]

Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 43.27%. Comparing base (07c1731) to head (b5e3760).
Report is 5 commits behind head on master.

❗ Current head b5e3760 differs from pull request most recent head f10a004

Please upload reports for the commit f10a004 to get more accurate results.

Files	Patch %	Lines
pkg/trafficrouting/manager.go	26.66%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #224      +/-   ##
==========================================
- Coverage   43.63%   43.27%   -0.37%     
==========================================
  Files          52       53       +1     
  Lines        5681     5849     +168     
==========================================
+ Hits         2479     2531      +52     
- Misses       2778     2877      +99     
- Partials      424      441      +17     

Flag	Coverage Δ
unittests	43.27% <26.66%> (-0.37%)	⬇️

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[zmberg]

@lujiajing1126 There is no problem with the code logic and implementation. We didn't implement it before mainly because we didn't think through the scenarios, so would it be convenient for you to participate in a community meeting? Time: 7.11. Alternatively, you can join the dingding group and we'll talk further!

[lujiajing1126]

@lujiajing1126 There is no problem with the code logic and implementation. We didn't implement it before mainly because we didn't think through the scenarios, so would it be convenient for you to participate in a community meeting? Time: 7.11. Alternatively, you can join the dingding group and we'll talk further!

Yes, sure. I've already in the Dingtalk group and will attend the community meeting on 11, Jul.

[lujiajing1126]

Follow the logic of CustomNetworkProvider here.

Gentle Ping @myname4423 @zmberg for review

[lujiajing1126]

Test cases have been moved to v1beta1 file and a workflow has been added. @myname4423

[zmberg]

/lgtm