feat: support custom webpack and ngx-env builders

[9kubczas4]

Description

This PR supports the custom Angular builders requested in the following issues:

• https://github.com/firebase/firebase-tools/issues/6779
• https://github.com/firebase/firebase-tools/issues/6602

Scenarios Tested

Deployed Angular applications which uses following builders:

• custom webpack
• ngx-env builder.

Sample Commands