Adding support for `sub` field

[kevinvalk]

Great library!

Could you add/expose sub field for the JWT fetching at:

web-auth-library/google/auth.ts

Line 68 in 437a11e

.btoa(JSON.stringify({ iss, aud, scope, exp, iat }))

That way we can use this to impersonate users through domain wide delegation.

Thanks a ton!