Update aggchain protobuf to rm the signature in `AggchainProof`

[hadjiszs]

Note

This isn't necessary nor critical but would make the protobuf cleaner.

Context

This issue relates to this signature:

interop/proto/agglayer/interop/types/v1/aggchain.proto

Lines 61 to 69 in e3e47eb

	// Important: The signature is ignored if the aggchain proof is part of a AggchainProofWithMultisig.
	// The signature linked to the aggchain-proof.
	// This signature is used to verify the authenticity of the aggchain-proof.
	// It represents a commitment to:
	// - `NewLocalExitRoot`
	// - `ClaimHash` (which is the hash of all the claims's global index + bridge exit hash)
	// - `Height` of the certificate associated to this aggchain-proof
	// - `AggchainParams` of the aggchain-proof
	FixedBytes65 signature = 2;

What we had until now

This path is used only for Katana for the FEP, so only one aggsender.
So far, this signature V4 was verified only on the agglayer node side.

What we have now

Since agglayer/agglayer#999:

• the trusted sequencer of the katana network will be registered as a multisig committee 1-of-1 in the L1
• this signature is verified in the PP
• this signature is expected to be V5 (katana's aggsender will be upgraded)

Consequently, in the agglayer and in the PP we consider this case as simply an AggchainProofWithMultisig.

To do

We now can afford cleaning this signature field in the message AggchainProof, because not needed anymore.
In terms of backward compatibility, it impacts only one aggsender (katana) which is expected to be upgraded.

[Freyskeyd]

An option is to use reserved