feat: enable more powerful vesting accounts

[JimLarson]

Expand the LockedCoins() method to take a full sdk.Context.

Description

Used in support of Agoric/agoric-sdk#3277 where a new type of "vesting account" will query to the Agoric swingset layer to see if there are any outstanding liens. Needs the full sdk.Context to do so.

Not marking as a breaking change, since this interface is not for general consumption.

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[michaelfig]

LGTM! Very clear.