feat(keyring-api): add token approval type

[hmalik88]

This adds a token approval type to TransactionType and subsequently TransactionStruct. The change was made as it is needed to make the distinction for transaction type in the UI, otherwise we see "Interaction"

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Note

Low Risk
Small additive change to an enum and its struct validation; risk is limited to downstream consumers needing to handle the new transaction type string.

Overview
Adds a new TransactionType.TokenApprove (token:approve) variant to represent ERC-20/asset approval transactions.

Updates TransactionStruct validation to accept the new type, and documents the addition in the keyring-api changelog under Unreleased.

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