Function get_proposal_changes 

pub fn get_proposal_changes(
    staged_commit: &StagedCommit,
    openmls_group: &MlsGroup,
    immutable_metadata: &GroupMetadata,
    mutable_metadata: &GroupMutableMetadata,
) -> Result<ProposalChanges, CommitRuleError>

Extracts the installations added and removed via proposals in the commit. Also returns a list of credentials from existing members that need verification (caused by update proposals) Tracks which proposer created each proposal for permission validation.