Remove remnants of bitcoin v0.29

[tvolk131]

Follow-up to #2356

We've migrated almost all of the codebase to bitcoin v0.30, but there are a few places remaining where we're using v0.29 which require some more care to remove:

1. PartiallySignedTransaction no longer implements Encodable or Decodable, which is why we're currently converting to/from the v0.29 version when encoding/decoding:

fedimint/fedimint-core/src/encoding/btc.rs

Lines 48 to 67 in 93c23e1

	impl crate::encoding::Encodable for bitcoin::psbt::PartiallySignedTransaction {
	fn consensus_encode<W: std::io::Write>(&self, writer: &mut W) -> Result<usize, std::io::Error> {
	bitcoin29::consensus::Encodable::consensus_encode(
	&bitcoin30_to_bitcoin29_psbt(self),
	writer,
	)
	}
	}
	impl crate::encoding::Decodable for bitcoin::psbt::PartiallySignedTransaction {
	fn consensus_decode_from_finite_reader<D: std::io::Read>(
	d: &mut D,
	_modules: &ModuleDecoderRegistry,
	) -> Result<Self, crate::encoding::DecodeError> {
	Ok(bitcoin29_to_bitcoin30_psbt(
	&bitcoin29::consensus::Decodable::consensus_decode_from_finite_reader(d)
	.map_err(crate::encoding::DecodeError::from_err)?,
	))
	}
	}

Ideally we would be able to use impl_encode_decode_bridge!(bitcoin::psbt::PartiallySignedTransaction); like we did prior to the migration to v0.30 but I'm not sure how feasible that is.

2. bitcoin::network::constants::Magic is encoded/decoded in reverse byte order to v29 (which doesn't have a struct, but instead just uses a u32). See here for details:

fedimint/fedimint-core/src/bitcoin_migration.rs

Lines 15 to 39 in 93c23e1

	pub fn bitcoin29_to_bitcoin30_network_magic(magic: u32) -> bitcoin::network::Magic {
	// Invert the byte order when converting from v0.29 to v0.30.
	// See the following bitcoin v0.29 and v0.30 code:
	// https://docs.rs/bitcoin/0.29.2/src/bitcoin/network/constants.rs.html#81-84
	// https://docs.rs/bitcoin/0.30.2/src/bitcoin/network/constants.rs.html#251-258
	let bytes = [
	(magic & 0xFF) as u8,
	((magic >> 8) & 0xFF) as u8,
	((magic >> 16) & 0xFF) as u8,
	((magic >> 24) & 0xFF) as u8,
	];
	bitcoin::network::Magic::from_bytes(bytes)
	}
	pub fn bitcoin30_to_bitcoin29_network_magic(magic: &bitcoin::network::Magic) -> u32 {
	let bytes = magic.to_bytes();
	// Invert the byte order when converting from v0.30 to v0.29.
	// See the following bitcoin v0.29 and v0.30 code:
	// https://docs.rs/bitcoin/0.29.2/src/bitcoin/network/constants.rs.html#81-84
	// https://docs.rs/bitcoin/0.30.2/src/bitcoin/network/constants.rs.html#251-258
	((bytes[3] as u32) << 24)
	| ((bytes[2] as u32) << 16)
	| ((bytes[1] as u32) << 8)
	| (bytes[0] as u32)
	}

3. bitcoin::hashes::hex::format_hex was removed in v30. For simplicity, we've simply left v29 around to rely on this single function. Let's find a way to retain this behavior without needing v29.