Add support for remote forges

tools/mothership/worker_server/forge/BUILD

@@ -0,0 +1,12 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "forge",
+    srcs = [
+        "caching.go",
+        "forge.go",
+    ],
+    importpath = "go.resf.org/peridot/tools/mothership/worker_server/forge",
+    visibility = ["//visibility:public"],
+    deps = ["//vendor/github.com/go-git/go-git/v5/plumbing/transport"],
+)

tools/mothership/worker_server/forge/caching.go

@@ -1 +1,35 @@
 package forge
+
+import "time"
+
+type Cacher struct {
+	Forge
+
+	authenticator *Authenticator
+}
+
+func NewCacher(f Forge) *Cacher {
+	return &Cacher{
+		Forge: f,
+	}
+}
+
+func (c *Cacher) GetAuthenticator() (*Authenticator, error) {
+	if c.authenticator != nil {
+		// Check if the token is expired, if not return it
+		if !c.authenticator.Expires.Before(time.Now()) {
+			return c.authenticator, nil
+		}
+	}
+
+	// Otherwise, get a new token
+	c.authenticator = nil
+	a, err := c.Forge.GetAuthenticator()
+	if err != nil {
+		return nil, err
+	}
+
+	c.authenticator = a
+
+	return a, nil
+}

tools/mothership/worker_server/forge/forge.go

@@ -1 +1,22 @@
 package forge
+
+import (
+	"github.com/go-git/go-git/v5/plumbing/transport"
+	"time"
+)
+
+type Authenticator struct {
+	transport.AuthMethod
+
+	AuthorName  string
+	AuthorEmail string
+	// Expires is the time when the token expires.
+	// So it can be used to cache the token.
+	Expires time.Time
+}
+
+type Forge interface {
+	GetAuthenticator() (*Authenticator, error)
+	GetRemote() string
+	EnsureRepositoryExists(auth *Authenticator, repo string) error
+}