handle property
The account's bidirectionally verified handle, or handleInvalid when it
has none that verifies.
Resolving from a handle rejects a DID document that does not claim it,
so this is always the supplied handle in that case. Resolving from a DID
reads the handle the document claims and checks that it resolves back to
that DID; anything else — no claim, a claim that resolves elsewhere, a
handle resolver that cannot be reached — is reported as handleInvalid,
because an account with a broken handle is still an account.
Modelled on com.atproto.identity.defs#identityInfo, which requires the
field and defines the same sentinel.
Implementation
final String handle;