feat(ooapi): add toplevel client and simplify API (#248)

* feat(ooapi): add toplevel client and simplify API

This diff should simplify using ooapi from other packages by
adding more abstraction that wraps the existing code.

Part of https://github.com/ooni/probe/issues/1355.

* fix(ooapi): use correct comment for cloners

See https://github.com/ooni/probe-cli/pull/248#discussion_r590663843

* fix(ooapi): make sure the documentation is current

See https://github.com/ooni/probe-cli/pull/248#discussion_r590665773

* fix(ooapi): automate copying APIs

See https://github.com/ooni/probe-cli/pull/248#discussion_r590665837

* feat(ooapi): add unit tests for clientcall.go

See https://github.com/ooni/probe-cli/pull/248#discussion_r590666297

* fix(ooapi): rewrite integration tests to use toplevel API

See https://github.com/ooni/probe-cli/pull/248#discussion_r590665084

internal/engine/netx/gocertifi/certifi.go

@@ -1,5 +1,5 @@
 // Code generated by go generate; DO NOT EDIT.
-// 2021-03-03 11:48:43.129132377 +0100 CET m=+2.301468593
+// 2021-03-10 12:20:35.042573349 +0100 CET m=+0.416591177
 // https://curl.haxx.se/ca/cacert.pem
 
 package gocertifi
@@ -3241,7 +3241,7 @@ kpzNNIaRkPpkUZ3+/uul9XXeifdy
 `
 
 // CACerts builds an X.509 certificate pool containing the
-// certificate bundle from https://curl.haxx.se/ca/cacert.pem fetch on 2021-03-03 11:48:43.129132377 +0100 CET m=+2.301468593.
+// certificate bundle from https://curl.haxx.se/ca/cacert.pem fetch on 2021-03-10 12:20:35.042573349 +0100 CET m=+0.416591177.
 // Returns nil on error along with an appropriate error code.
 func CACerts() (*x509.CertPool, error) {
 	pool := x509.NewCertPool()