Obtain an access token.

https://developer.investec.com/programmable-banking/#get-access-token

=> (access-token "client-id" "secret")
{:access-token "Ms9OsZkyrhBZd5yQJgfEtiDy4t2c"
 :token-type "Bearer"
 :expires-in #object[java.time.OffsetDateTime 0x4304ab94 "2022-02-17T04:17:54.662519+02:00"]
 :scope "accounts"}