Refactor/foreign assets as thin proxy #2221

Job	Run time
collator-selection	0s
canary	0s
forkless-update-data	0s
forkless-update-no-data	0s
governance	0s
node-only-update	0s
try-runtime	0s
codestyle / rustfmt	16s
codestyle / yarn_eslint	28s
unit-test / develop	13m 57s
codestyle / clippy	2m 11s
xcm / Prepare execution matrix	45s
yarn-dev / opal	40m 5s
yarn-dev / sapphire	48m 34s
yarn-dev / quartz	48m 58s
yarn-dev / unique	33m 39s
xcm / opal	1h 21m 20s
xcm / quartz	2h 48m 46s
xcm / unique	2h 58m 10s
	10h 17m 9s

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