cli: add derived loader

[gusinacio]

This is the continuation of the PR graphprotocol/graph-node#4434 and is related to graphprotocol/graph-node#4004

Thanks to @poonai for part of the code from #977.

What does this PR do?

• adds loadRelated(entity, id, field) to the host
• creates a <Entity>Loader generator for accessing load() in a more explicit way
• fix the tests related to derived fields

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Latest commit: 2db3719

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[azf20]

hey @flametuner checking in on this, @saihaj this is complementary to some exciting new Graph Node functionality (graphprotocol/graph-node#4434) so would be great to get this over the line!

[neysofu]

This feature will be shipped in graph-node v0.31.0, but I'm omitting it from the release notes because it's not usable without this PR. Making a note here so we'll remember to announce it in v0.32.0.

cc @incrypto32

[azf20]

Thanks @flametuner - closing in favour of #1340