Improved Transaction Decoding

[bh2smith]

Most of the heavy lifting was introduced in the direct dependency (BitteProtocol/near-ca#146).
Here we use that to improve our transaction decoding functionality:

• Add Type Guard for UserOperation
• Inherit Type Guards from near-ca
• improve decodeTx function safety
• Unit test isUserOperation
• Ensure the logged Warning disappears in existing test (adapter: encodeEvmTx)

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