feat(upward): add UrlResolver and tests. Closes #1057

[zetlen]

Description

Add an eighth Resolver to UPWARD, the UrlResolver.

• Add the resolver to upward-js, along with unit tests.
• Add tests for the UrlResolver to upward-spec, the cross-platform test suite.
• Add documentation to the README for UPWARD.
• Modify venia-upward.yml to use the UrlResolver instead of TempateResolvers to join URLs.

Related Issue

Closes #1057.
Related to #1045 and other issues resolving URLs for ServiceResolvers or ProxyResolvers..

Motivation and Context

See #1057; URLs should be a first class concept in UPWARD.

Verification

Experiment with URL resolution based on the documentation. Try different configurations in Venia.

How Has This Been Tested?

Full test coverage and integration test suite coverage.
Tried all cases in Venia where UPWARD resolves URLs. This resolver simplifies the Venia UPWARD definition and makes it more reliable.

Proposed Labels for Change Type/Package

FEATURE in upward-js
ENHANCEMENT in venia-concept

Checklist:

• I have read the CONTRIBUTING document.
• I have linked an issue to this PR.
• I have indicated the change type and relevant package(s).
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All CI checks are green (linting, build/deploy, etc).
• At least one core contributor has approved this PR.

[vercel]

This pull request is automatically deployed with Now.
To access deployments, click Details below or on the icon next to each push.

[jcalcaben]

The docs look good to me. Unassigning myself so others can pick this up and review the implementation.

[tjwiebell]

Looks good, some minor changes should make this a little easier to understand. If we've got sign off from the team that this is a priority, I can jump on the upward-php implementation, so start brushing up on your PHP so you can review that for me.

[tjwiebell]

@zetlen - Ran a quick validation of this strategy, and we can move the venia-concept change to a separate PR that waits for upward-php, and merge the rest now. This would also allow us to publish an alpha release of upward-spec so I can more easily test and write the upward-php implementation.

What do you think of that approach?

[coveralls]

Coverage increased (+0.4%) to 76.822% when pulling 666dfa6 on zetlen/upward-add-url-resolver into 6129afe on develop.

[zetlen]

@magento-research/core-pwa-devs Any other concerns before we merge?