added mixpanel headers to /init api endpoint

[mdeshpande12]

• Updated .env.sample file
• Updated configOverride.js file so that we can extract the package.json version and webchat version from env file; save it into variables and expose them to the code in the build file.
• Updated test file

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
96.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud