Build system: Add CDN-based, Cross Origin Storage ready Prebid distribution

[dgirardi]

Type of change

• Feature

Description of change

This adds some tooling to load prebid from a CDN in a way that should be compatible with cross origin storage. The build now generates:

• prebid.web.js: this is intended as a drop-in replacement for the "normal" prebid.js bundle. Instead of being a concatenation of chunks selected with the build's --modules, it contains only the names and checksums of those chunks, and adds correspoding <script> tags to load them from a CDN (by default this is https://cdn.jsdelivr.net/npm/prebid.js@<version>/dist/chunks/, controllable with --distUrlBase).
  Note that the default distUrlBase points to a specific version only when building a tagged version (the version in package.json does not end with '-pre'). When building master it points to latest, which is unlikely to have matching checksums; the generated prebid.web.js would fail to load in that case.
• bundle.js: this is intended to be loaded directly from a CDN. Once released, publishers should be able to load prebid with

    <script async src="https://cdn.jsdelivr.net/npm/prebid.js@<version>/dist/bundle.js?modules=<modules>"></script>

  it works first by loading a manifest file that contains chunk names, their checksum, and their dependencies; resolving the chunks needed for the given modules, then adding a <script> tag for each chunk.
  It supports setting the globalVarName and distUrlBase build options (e.g. bundle.js?globalVarName=customGlobal), but not defineGlobal, as a global variable is the only way to interact with Prebdi when loaded this way.

Other information

Closes #15256

[github-actions]

This PR introduces changes that may not work on all browsers. According to Babel, the following polyfills may be needed, and they are not automatically included:

• Changes to test/build-logic/web-bundler/manifest_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/dependencies_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/load_spec.js may need:
  • es.iterator.constructor
  • es.iterator.for-each
  • esnext.iterator.constructor
  • esnext.iterator.for-each

The best way to address this is to provide good test coverage, as normal PR checks run unit tests on older browsers.

[github-actions]

Tread carefully! This PR adds 2 linter errors (possibly disabled through directives):

• test/spec/web-bundler/load_spec.js (+2 errors)

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[github-actions]

This PR introduces changes that may not work on all browsers. According to Babel, the following polyfills may be needed, and they are not automatically included:

• Changes to test/build-logic/manifest_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/dependencies_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/load_spec.js may need:
  • es.iterator.constructor
  • es.iterator.for-each
  • esnext.iterator.constructor
  • esnext.iterator.for-each

The best way to address this is to provide good test coverage, as normal PR checks run unit tests on older browsers.

[barecheck]

Barecheck - Code coverage report

Total: 96.56%

Your code coverage diff: 0.00% ▴

Uncovered files and lines

File	Lines
test/spec/web-bundler/load_spec.js	74

[github-actions]

This PR introduces changes that may not work on all browsers. According to Babel, the following polyfills may be needed, and they are not automatically included:

• Changes to test/build-logic/manifest_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/dependencies_spec.js may need:
  • es.array.push
• Changes to test/spec/web-bundler/load_spec.js may need:
  • es.iterator.constructor
  • es.iterator.for-each
  • esnext.iterator.constructor
  • esnext.iterator.for-each

The best way to address this is to provide good test coverage, as normal PR checks run unit tests on older browsers.