polywrap.build.yaml - allow developer to use a custom polywrap-build.sh script

[dOrgJelli]

Is your feature request related to a problem? Please describe. When developers are building wasm wrappers with the vm build strategy, the only way to customize it is to modify the build script located at .polywrap/wasm/build/project/polywrap-build.sh directory, and run the docker command manually.

Describe the solution you'd like Developers should be able to set the build script within the build manifest, something similar to this:

strategies:
  vm:
    build_script: ./custom.sh

[dOrgJelli]

This has been reported as a required feature by one of our users, who needs to pass in a "prod" environment variables at build-time.

[dOrgJelli]

I've started work on this, will push shortly.