Refactor/foreign assets as thin proxy #2203

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
unit-test / develop	7m 39s
xcm / Prepare execution matrix	30s
yarn-dev / opal	9m 48s
yarn-dev / sapphire	9m 46s
yarn-dev / quartz	9m 46s
yarn-dev / unique	10m 53s
codestyle / rustfmt	26s
codestyle / yarn_eslint	46s
codestyle / clippy	2m 59s
xcm / opal	9m 12s
xcm / quartz	9m 13s
xcm / unique	9m 11s
	1h 20m 9s

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