Added android build script

exported the mls_create_keypackage as uniffi

fixed bug in creating multiple groups

updated mls_get_group_members function

code refactored

no need to serialize MLSGroup. Can use the StorageProvider to store the group

added new tests for testing the api functions (exported)

added new tests for testing the api functions (exported)

updated cargo deps

refrectored code

implemented helper function for getting members by username

implemented function for processing commit message after adding a new member to the group

implemented test functioalities for adding a third member to the group

facilitated key store json file

added prefix to api method names