feat(group): add message expiration to group metadata

[mchenani]

This pull request adds support for message expiration policies in group settings. The key updates include:
• A new message_expiration_ms field added to group creation options: holds how long after the message was received gets deleted
• A new message_expiration_from_ms field added to group creation options: holds from when the messages get deleted
• Permission updates to allow managing message expiration settings.

#1449

[mchenani]

Still thinking of setting this to null instead of 0, if you have another idea, lmk.

[codabrink]

I would. And just wrap the insert logic down below with a

if let Some(message_expiration_ms) = opts.message_expiration_ms {
  // insert...
}

Putting in a zero just seems like we're emulating null-state when we don't have to.

[nplasterer]

@cameronvoell might have thoughts on this.

[mchenani]

Since its a value in the map, instead of keeping a default value I decided to not insert it at all.

[nplasterer]

We had issues in the past of if two people are using two different versions of the SDK in the same group chat. We need to make sure that the person on the old version of the sdk does not break. I believe there was some sort of migration that needs to happen but again @cameronvoell would remember better than me how we decided to go about this for new metadata.

[cameronvoell]

No migration needed here, existing groups will not have message expiration set, but updating the expiration set will be possible with default permission of admin_only (for groups created before this change goes in) =>

libxmtp/xmtp_mls/src/groups/group_permissions.rs

Lines 1041 to 1049 in ab9bb05

	// Policy is not found for metadata change, let's check if the new field contains the super_admin prefix
	// and evaluate accordingly
	let policy_for_unrecognized_field =
	if change.field_name.starts_with(SUPER_ADMIN_METADATA_PREFIX) {
	MetadataPolicies::allow_if_actor_super_admin()
	} else {
	// Otherwise we default to admin only for fields with missing policies
	MetadataPolicies::allow_if_actor_admin()
	};

That path of adding new metadata to existing groups was tested with this change here, which we can reference for adding expiration in this pr.

[mchenani]

I Added only one policy for both message_expiration_ms and message_expiration_from_ms. wdyt?

[cameronvoell]

yea I think that makes sense, doesnt make sense for someone to be able to change one without the other 👍