Refactor/foreign assets as thin proxy #2205

Job	Run time
canary	0s
collator-selection	0s
forkless-update-data	0s
forkless-update-no-data	0s
try-runtime	0s
governance	0s
node-only-update	0s
xcm / Prepare execution matrix	14s
unit-test / develop	7m 33s
yarn-dev / opal	39m 18s
yarn-dev / sapphire	27m 46s
yarn-dev / quartz	28m 8s
yarn-dev / unique	27m 59s
codestyle / rustfmt	14s
codestyle / clippy	2m 33s
codestyle / yarn_eslint	25s
xcm / opal	1h 21m 6s
xcm / quartz	1h 46m 19s
xcm / unique	1h 46m 20s
	7h 7m 55s

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