feature!: support for setting HTTP User-Agent header (#157)

* feature!: support for setting HTTP User-Agent header

* fix lint

* fix e2e

* refactor: move http.go to internal/util/useragent package and rename functions to Get and Set

* Move packages and use attest version

internal/useragent/useragent_test.go

@@ -0,0 +1,19 @@
+package useragent
+
+import (
+	"context"
+	"testing"
+)
+
+// test the user agent setting and getting.
+func TestSetUserAgent(t *testing.T) {
+	ctx := context.Background()
+	if Get(ctx) != defaultUserAgent {
+		t.Errorf("expected user agent to be '%s', got %q", defaultUserAgent, Get(ctx))
+	}
+
+	ctx = Set(ctx, "test-agent")
+	if Get(ctx) != "test-agent" {
+		t.Errorf("expected user agent to be 'test-agent', got %q", Get(ctx))
+	}
+}