feat(publisher-ers): support flavor config

[erickzhao]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Closes #1215

Adds support for the flavor config in electron-release-server. This is an optional parameter for the ERS publisher. Also added a set of basic tests for the publisher.

[codecov]

Codecov Report

Merging #2766 (784f669) into master (9026eae) will decrease coverage by 47.66%.
The diff coverage is 100.00%.

❗ Current head 784f669 differs from pull request most recent head eb0d2d0. Consider uploading reports for the commit eb0d2d0 to get more accurate results

@@             Coverage Diff             @@
##           master    #2766       +/-   ##
===========================================
- Coverage   73.93%   26.27%   -47.67%     
===========================================
  Files          77       73        -4     
  Lines        2356     2253      -103     
  Branches      436      423       -13     
===========================================
- Hits         1742      592     -1150     
- Misses        464     1654     +1190     
+ Partials      150        7      -143     

Impacted Files	Coverage Δ
...lisher/electron-release-server/src/PublisherERS.ts	86.76% <100.00%> (+59.49%)	⬆️
...s/plugin/local-electron/src/LocalElectronPlugin.ts	10.00% <0.00%> (-90.00%)	⬇️
packages/api/core/src/util/upgrade-forge-config.ts	10.52% <0.00%> (-89.48%)	⬇️
packages/plugin/webpack/src/util/once.ts	11.11% <0.00%> (-88.89%)	⬇️
packages/maker/wix/src/util/author-name.ts	20.00% <0.00%> (-80.00%)	⬇️
packages/maker/appx/src/util/author-name.ts	20.00% <0.00%> (-80.00%)	⬇️
...ypescript-webpack/src/TypeScriptWebpackTemplate.ts	20.00% <0.00%> (-80.00%)	⬇️
packages/plugin/webpack/src/WebpackConfig.ts	18.98% <0.00%> (-79.75%)	⬇️
packages/api/core/src/util/publish-state.ts	12.82% <0.00%> (-79.49%)	⬇️
packages/api/core/src/util/rebuild.ts	16.66% <0.00%> (-78.08%)	⬇️
... and 57 more

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