Cosmos: Allow Resolving Container and Database by RID

[analogrelay]

We should allow creating a DatabaseClient and ContainerClient using RIDs instead of names. Before GA, we should consider introducing an enum like ResourceIdentity (name TBD):

#[non_exhaustive]
pub enum ResourceIdentity<'a> {
    Name(&'a str),
    Rid(&'a ResourceId),
}

Then we can change database_client and container_client APIs to take impl Into<ResourceIdentity<'_>> and implement Into<ResourceIdentity> for &str and a ResourceId newtype.