exchangeCode(params: URLSearchParams): Promise<{ id_token: string; token_type: string; }>
Trades a code for an identity token, for POST /api/oauth/tokens, which
the client calls server to server with the form params. A code works once,
within a minute, for the client and redirect it was made for, and with the
verifier whose hash the request carried (PKCE). The token, signed with the
instance key and addressed to the client, carries identityClaims
and the request's nonce.
params: URLSearchParams