Struct for mapping result of olm_rs::account::OlmAccount::one_time_keys()
Currently we manually deconstruct the JSON object returned by the olm_rs::account::OlmAccount::one_time_keys()
function.
This can be seen here: https://gitlab.gnome.org/jhaye/metaolm/blob/master/src/otk_replenisher.rs#L78
Instead it would be better to use a struct that we then map the JSON value against using serde.