Verify the X-Langsmith-User-Token header of a service URL request.
LangSmith sets this header only for service URLs that use LangSmith
login. Code that knows it is running in a sandbox can instead trust the
unsigned X-Langsmith-User-Id and X-Langsmith-User-Email headers,
which the sandbox runtime strips from inbound requests and sets itself;
verify this token when that is not guaranteed.
The header value.
The service URL host the request was sent to, such as
request.headers["Host"]. A full service URL is also accepted.
If set, the LangSmith app URL the token must be issued by.
