Add the serialized transaction size to UnminedTx

[teor2345]

Motivation

ZIP-401 weights transactions by their serialized size.

Consensus Rules

The mempool of a node holds a set of transactions.
Each transaction has a cost, which is an integer defined as:
max(serialized transaction size in bytes, 4000)

https://zips.z.cash/zip-0401#specification

Design

This existing code calculates the size of a Message:

zebra/zebra-network/src/protocol/external/codec.rs

Lines 176 to 183 in de589f6

	/// Obtain the size of the body of a given message. This will match the
	/// number of bytes written to the writer provided to `write_body` for the
	/// same message.
	///
	/// TODO: Replace with a size estimate, to avoid multiple serializations
	/// for large data structures like lists, blocks, and transactions.
	/// See #1774.
	fn body_length(&self, msg: &Message) -> usize {

Maybe we want to turn it into a default method on the ZcashSerialize trait?

[teor2345]

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