Deposit to enable unknown-tokens

[xlc]

People needs to pay some deposit to allow them receive unknown tokens. X tokens per token type. We need to track how many unique unknown tokens a user have.

Needs an update_enabled_count(origin, count: u32) call to allow user to enable / disable (by setting count to zero) unknown tokens.

Relates: https://github.com/paritytech/polkadot/issues/2702