txscript: allow multiple push operations for OP_RETURN

[EricGrill]

Summary

This PR updates the isNullDataScript function to allow multiple data push operations after OP_RETURN, matching Bitcoin Core's behavior as of v0.12.0 (2016) which removed the single push restriction.

Changes:

• Add MaxNullDataScriptSize constant (10000 bytes) for total script size limit
• Keep MaxDataCarrierSize at 80 bytes for backwards compatibility with NullDataScript function callers
• Update isNullDataScript to use IsPushOnlyScript to validate that all opcodes after OP_RETURN are valid data pushes (OP_0 through OP_16 and direct/PUSHDATA pushes)
• Enforce limit on total script size instead of individual push size
• Update and add test cases for the new behavior

Technical Details:

Previously, btcd only accepted:

• OP_RETURN alone
• OP_RETURN + single data push (up to 80 bytes)

After this change, btcd accepts:

• OP_RETURN alone
• OP_RETURN + any number of push-only opcodes (total script size <= 10000 bytes)

This matches Bitcoin Core's Solver function which checks:

if (scriptPubKey.size() >= 1 && scriptPubKey[0] == OP_RETURN && 
    scriptPubKey.IsPushOnly(scriptPubKey.begin()+1))

Use Cases:

This allows protocols like Metaprotocol, Counterparty, and others that use multiple data pushes in OP_RETURN outputs to be recognized as standard null data scripts.

Test plan

• Updated existing test cases to reflect new behavior
• Added test cases for multiple push operations
• Added test case for non-push opcode after OP_RETURN (should remain NonStandard)
• Added test cases for scripts at and exceeding the 10000 byte limit
• Run full test suite (requires Go installation)

Fixes #2305

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