wallet-ext: dapp interface method to request for coins with specific balance

[pchrysochoidis]

add a requestCoin method (for SUI only for now if makes things simpler) to allow dapps to easily get a reference of coin with specified balance. If the coin does not exist but the user has enough balance the wallet can split/merge and return the new coin.

[github-actions[bot]]

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[Jordan-Mysten]

This shouldn't be needed with new rpc methods.