feat(demo): implement external router in-memory driver

[griest024]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the new behavior?

Demo implements external router in-memory driver behavior. The resolver in the passed config searches the product and category backends for entities with matching URLs.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Relies on #1841, #1842, #1838

[griest024]

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