feat(private): add server artifacts

[trivikr]

Issue

Server artifacts can now be added as support for Node.js 12.x ended in #4123

Description

Adds server artifacts in private workspace

Testing

Verified that:

• Server protocol tests are run in CodeBuild CI, and are successful

  lerna info run Ran npm script 'test' in '@aws-sdk/aws-restjson-server' in 17.1s:
  ...
  lerna info run Ran npm script 'test' in '@aws-sdk/aws-restjson-validation-server' in 7.1s:

• Server artifacts are updated when yarn generate-clients is called without option n
• Server artifacts have "private": true which ensures they won't be mistakenly published

Additional Context

The generic client aws-echo-service was added in #2950

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