Proposer: add proposerID to the shred format

[MaxResnick]

The existing shred layout looks like:

Offset	Size (bytes)	Field	Description
0	64	signature	Ed25519 signature
64	1	shred_variant	Variant byte (0b10??_????) for MerkleData
65	8	slot	Slot number (u64, little-endian)
73	4	index	Shred index (u32, little-endian)
77	2	version	Shred version (u16, little-endian)
79	4	fec_set_index	FEC set index (u32, little-endian)
83	2	parent_offset	Parent slot offset (u16, little-endian)
85	1	flags	ShredFlags bitfield
86	2	size	Total size of headers + data (u16, little-endian)
88	variable	data	Actual ledger entry data
...	32 (optional)	chained_merkle_root	Previous batch’s Merkle root (if chained)
...	variable	merkle_proof	Merkle proof entries (proof_size × 20 bytes)
...	64 (optional)	retransmitter_signature	Retransmitter signature (if resigned)

Offset	Size (bytes)	Field	Description
0	64	signature	Ed25519 signature
64	1	shred_variant	Variant byte (0b01??_????) for MerkleCode
65	8	slot	Slot number (u64, little-endian)
73	4	index	Shred index (u32, little-endian)
77	2	version	Shred version (u16, little-endian)
79	4	fec_set_index	FEC set index (u32, little-endian)
83	2	num_data_shreds	Number of data shreds in FEC set (u16, little-endian)
85	2	num_coding_shreds	Number of coding shreds in FEC set (u16, little-endian)
87	2	position	Position within coding shreds (u16, little-endian)
89	variable	erasure_coded_data	Erasure-coded shard data
...	32 (optional)	chained_merkle_root	Previous batch’s Merkle root (if chained)
...	variable	merkle_proof	Merkle proof entries (proof_size × 20 bytes)
...	64 (optional)	retransmitter_signature	Retransmitter signature (if resigned)

We need to add a proposer identifier to the shreds since we now have multiple proposers per slot