Return account's asset balances (#13352)

* Runtime method to get user's assets balances

* Fix test (typo)

* Update frame/assets/src/functions.rs

* Remove instance param

* Update frame/assets/src/functions.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Remove instance param

* Refactor

* Chore

* Update doc

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: parity-processbot <>