Chore: Add devenv block for Spanner

devenv/docker/blocks/spanner_tests/README.md

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+This devenv block spawns a Spanner emulator for Grafana integration tests and development purposes:
+
+```
+make devenv sources=spanner_tests
+```
+
+You should then be able to connect to the database using Go SQL Spanner driver. Here's a minimal example:
+
+```go
+package main
+
+import (
+	"log"
+
+	_ "github.com/googleapis/go-sql-spanner"
+
+	"context"
+	"database/sql"
+	"fmt"
+)
+
+func main() {
+	ctx := context.Background()
+	db, err := sql.Open("spanner", fmt.Sprintf("projects/%s/instances/%s/databases/%s", "grafanatest", "grafanatest", "grafanatest"))
+	if err != nil {
+		log.Fatalf("failed to open database connection: %v", err)
+	}
+	defer db.Close()
+
+	rows, err := db.QueryContext(ctx, "SELECT 'Hello World!'")
+	if err != nil {
+		log.Fatalf("failed to execute query: %v", err)
+	}
+	defer rows.Close()
+
+	var msg string
+	for rows.Next() {
+		if err := rows.Scan(&msg); err != nil {
+			log.Fatalf("failed to scan row values: %v", err)
+		}
+		fmt.Printf("%s\n", msg)
+	}
+	if err := rows.Err(); err != nil {
+		log.Fatalf("failed to execute query: %v", err)
+	}
+}
+```
+
+Run it and you should see the "Hello world" message:
+
+```
+SPANNER_EMULATOR_HOST=localhost:9010 go run example.go
+```

devenv/docker/blocks/spanner_tests/docker-compose.yaml

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+  spannertests:
+    image: gcr.io/cloud-spanner-emulator/emulator:1.5.24
+    ports:
+      - "9010:9010"
+      - "9020:9020"
+  spannertests_gcloud:
+    image: google/cloud-sdk:509.0.0
+    environment:
+      SPANNER_EMULATOR_HOST: spannertests:9010
+    command:
+      - /bin/sh
+      - -c
+      - |
+        sleep 5
+        gcloud config set auth/disable_credentials true
+        gcloud config set project grafanatest
+        gcloud config set api_endpoint_overrides/spanner http://spannertests:9020/
+        gcloud spanner instances create grafanatest --config=emulator-config --description="For integration tests" --nodes=1
+        gcloud spanner databases create --instance grafanatest grafanatest