feat(iroh-gossip): configure the max message size

[ramfox]

Description

Add configuration option for max_message_size for iroh-gossip::proto::Config.

This Config gets used in iroh-gossip::Gossip::from_endpoint.

iroh-docs still uses the default 4096 bytes. The max_message_size configuration is useful for folks using iroh-gossip::Gossip as its own library.

closes #2312

Breaking Changes

Adds:
iroh-gossip::Gossip::max_message_size - that reports the configured maximum message size for the gossip actor.

Changes:
iroh_gossip::net::util::read_message now takes a max_message_size: usize parameter
iroh_gossip::net::util::write_message now takes a max_message_size: usize parameter
iroh_gossip::net::util::read_lp now takes a max_message_size: usize parameter

Removes:
iroh-gossip::proto:: MAX_MESSAGE_SIZE const

Change checklist

• Self-review.
• Documentation updates if relevant.
• All breaking changes documented.

[divagant-martian]

LGTM, maybe add some comments about a couple of caveats.

For example:

• This should be a network-wide configuration parameter to ensure all nodes can transmit and read large messages.
• This should be large enough to allow for gossip control messages to be sent.
• Any other we can think of