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/ooapi/client.go

@@ -0,0 +1,13 @@
+package ooapi
+
+// Client is a client for speaking with the OONI API. Make sure you
+// fill in the mandatory fields.
+type Client struct {
+	BaseURL      string       // optional
+	GobCodec     GobCodec     // optional
+	HTTPClient   HTTPClient   // optional
+	JSONCodec    JSONCodec    // optional
+	KVStore      KVStore      // mandatory
+	RequestMaker RequestMaker // optional
+	UserAgent    string       // optional
+}