Investigate ClientConfig.key_package_lifetime being used for cases other than KP creation.

[CaioSym]

Problem:

Splitting this from the discussion in https://github.com/awslabs/mls-rs/pull/196/files#r1786752471

The current implementation of mls-rs defines a ClientConfig.key_package_lifetime parameter which is intended to be used when creating KPs. Internally, however, this parameter seems to be used whenever a KemTree::Node is created. For example, when calling Client::create_group, that calls into Group::new which calls LeafNode::generate(..., config.lifetime())```. ClientConfig::lifetimeunder the hood uses theClientConfig.settings.lifetime_in_sto compute the returned value.ClientConfig.settings.lifetime_in_sis the backing value ofClientConfig.key_package_lifetime`.

Note that this is not the only place where ClientConfig::lifetime is passed to LeafNode::generate or other LN functions

Solution:

Potential solutions here are:

• Renaming ClientConfig.key_package_lifetime and ClientConfig.key_package_not_before to something that indicates these are parameters driving the Node lifetimes instead. Ie ClientConfig.tree_node_lifetime and ClientConfig.tree_node_not_before
• Splitting the lifetime params used for group creation / other non KP scenarios from the KP lifetimes. Extract the KP lifetimes to be parameters passed during KP generation.
• Some other refactor?
• Keep existing api to avoid breaking changes, Provide documentation to address potential confusion.

Requirements / Acceptance Criteria:

TBD based on what solution we chose

Out of scope:

N/A