Refactor/foreign assets as thin proxy #2223

Job	Run time
collator-selection	0s
forkless-update-data	0s
canary	0s
forkless-update-no-data	0s
governance	0s
node-only-update	0s
try-runtime	0s
codestyle / yarn_eslint	1m 8s
codestyle / clippy	2m 40s
codestyle / rustfmt	14s
unit-test / develop	7m 26s
xcm / Prepare execution matrix	14s
yarn-dev / opal	24m 44s
yarn-dev / sapphire	41m 17s
yarn-dev / quartz	40m 56s
yarn-dev / unique	26m 52s
xcm / opal	1h 19m 17s
xcm / quartz	2h 45m 24s
xcm / unique	2h 57m 28s
	9h 27m 40s

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