Add Expected Decimal Information to TokenMint, TokenBurn, and TokenWipeAccount

[mehcode]

Abstract

This specification proposes a way to add expected decimal information for tokens to
the TokenMint, TokenBurn, and TokenWipeAccount transaction.

Motivation

The motivation for this HIP is identical to the motivation for HIP-31.
These additional transactions also need the decimal information for use in a hardware wallet.

Rationale

The below change to token transfer protobuf messages is formulated to incur a minimal operational cost to the network while providing enough information to address hardware wallet security concerns.

Specification

message TokenMintTransactionBody {
    // [...]

    /**
     * If present, the number of decimals this fungible token type is expected to have. The transfer
     * will fail with UNEXPECTED_TOKEN_DECIMALS if the actual decimals differ.
     */
    UInt32Value expected_decimals = 4;
}

message TokenBurnTransactionBody {
    // [...]

    /**
     * If present, the number of decimals this fungible token type is expected to have. The transfer
     * will fail with UNEXPECTED_TOKEN_DECIMALS if the actual decimals differ.
     */
    UInt32Value expected_decimals = 4;
}

message TokenWipeAccountTransactionBody {
    // [...]

    /**
     * If present, the number of decimals this fungible token type is expected to have. The transfer
     * will fail with UNEXPECTED_TOKEN_DECIMALS if the actual decimals differ.
     */
    UInt32Value expected_decimals = 5;
}