Change the behaviour of setting up env variables

[prernagp]

Is your feature request related to a problem? Please describe.
At present, environment variables are set through environment.ts files included in the angular build package. However, we can rewrite the logic of injecting values on runtime, which can be helpful when the angular application is built using Docker.

Describe the solution you'd like
We can have a combination of both scenarios, where env variables are injected as a part of the build package or on runtime. We can create a mechanism to read the values from buildSpec file and replace them with the values in environment.ts according to the application environment i.e. QA, PROD pr STAGE. Refer to https://github.com/sourcefuse/soundplay-ui/pull/222