engine/ooapi: autogenerated API with login and caching (#234)

* internal/engine/ooapi: auto-generated API client

* feat: introduce the callers abstraction

* feat: implement API caching on disk

* feat: implement cloneWithToken when we require login

* feat: implement login

* fix: do not cache all APIs

* feat: start making space for more tests

* feat: implement caching policy

* feat: write tests for caching layer

* feat: add integration tests and fix some minor issues

* feat: write much more unit tests

* feat: add some more easy unit tests

* feat: add tests that use a local server

While there, make sure many fields we care about are OK.

* doc: write basic documentation

* fix: tweak sentence

* doc: improve ooapi documentation

* doc(ooapi): other documentation improvements

* fix(ooapi): remove caching for most APIs

We discussed this topic yesterday with @FedericoCeratto. The only
place where we want LRU caching is MeasurementMeta.

* feat(ooapi): improve handling of errors during login

This was also discussed yesterday with @FedericoCeratto

* fix(swaggerdiff_test.go): temporarily disable

Before I work on this, I need to tend onto other tasks.

* fix(ootest): add one more test case

We're going towards 100% coverage of this package, as it ought to be.

* feat(ooapi): test cases for when the probe clock is off

* fix(ooapi): change test to have 100% unittest coverage

* feat: sync server and client APIs definition

Companion PR: https://github.com/ooni/api/pull/218

* fix(ooapi): start testing again against API

* fix(ooapi): only generate each file once

* chore: set version to 3.7.0-alpha

While there, make sure we don't always skip a currently failing
riseupvpn test, and slightly clarify the readme.

* fix(kvstore): less scoped error message

internal/engine/ooapi/dependencies.go

@@ -0,0 +1,54 @@
+package ooapi
+
+import (
+	"context"
+	"io"
+	"net/http"
+)
+
+// JSONCodec is a JSON encoder and decoder.
+type JSONCodec interface {
+	// Encode encodes v as a serialized JSON byte slice.
+	Encode(v interface{}) ([]byte, error)
+
+	// Decode decodes the serialized JSON byte slice into v.
+	Decode(b []byte, v interface{}) error
+}
+
+// RequestMaker makes an HTTP request.
+type RequestMaker interface {
+	// NewRequest creates a new HTTP request.
+	NewRequest(ctx context.Context, method, URL string, body io.Reader) (*http.Request, error)
+}
+
+// TemplateExecutor parses and executes a text template.
+type TemplateExecutor interface {
+	// Execute takes in input a template string and some piece of data. It
+	// returns either a string where template parameters have been replaced,
+	// on success, or an error, on failure.
+	Execute(tmpl string, v interface{}) (string, error)
+}
+
+// HTTPClient is the interface of a generic HTTP client.
+type HTTPClient interface {
+	// Do should work like http.Client.Do.
+	Do(req *http.Request) (*http.Response, error)
+}
+
+// GobCodec is a Gob encoder and decoder.
+type GobCodec interface {
+	// Encode encodes v as a serialized gob byte slice.
+	Encode(v interface{}) ([]byte, error)
+
+	// Decode decodes the serialized gob byte slice into v.
+	Decode(b []byte, v interface{}) error
+}
+
+// KVStore is a key-value store.
+type KVStore interface {
+	// Get gets a value from the key-value store.
+	Get(key string) ([]byte, error)
+
+	// Set stores a value into the key-value store.
+	Set(key string, value []byte) error
+}