feat: create specified config files that do not exist (#1618)

Fixes: #1561

```
$ rm ftl-project-idk.toml
$ ftl config set yo --inline value -C ftl-project-idk.toml
warn: Creating a new project config file at "ftl-project-idk.toml" because the file does not already exist
$ ftl config set yo --inline value -C ftl-project-idk.toml
$ stat ftl-project-idk.toml                         
16777232 3291117 -rw------- 1 dli staff 0 125 "Jun  3 22:44:16 2024" "Jun  3 22:44:15 2024" "Jun  3 22:44:15 2024" "Jun  3 22:44:15 2024" 4096 8 0 ftl-project-idk.toml
$
```

cmd/ftl/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"net/url"
 	"os"
 	"os/signal"
@@ -83,7 +84,7 @@ func main() {
 	ctx = log.ContextWithLogger(ctx, logger)
 
 	config, err := projectconfig.LoadConfig(ctx, cli.ConfigFlag)
-	if err != nil {
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		kctx.Fatalf(err.Error())
 	}
 	kctx.Bind(config)

common/projectconfig/integration_test.go

@@ -0,0 +1,17 @@
+//go:build integration
+
+package projectconfig
+
+import (
+	"testing"
+
+	in "github.com/TBD54566975/ftl/integration"
+)
+
+func TestCmdsCreateProjectTomlFilesIfNonexistent(t *testing.T) {
+	in.Run(t, "",
+		in.CopyModule("echo"),
+		in.Exec("ftl", "config", "set", "key", "--inline", "value", "--config", "ftl-project-nonexistent.toml"),
+		in.FileExists("ftl-project-nonexistent.toml"),
+	)
+}

common/projectconfig/projectconfig.go

@@ -2,6 +2,7 @@ package projectconfig
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -75,6 +76,16 @@ func LoadWritableConfig(ctx context.Context, input []string) (Config, error) {
 		return Config{}, nil
 	}
 	target := configPaths[len(configPaths)-1]
+
+	logger := log.FromContext(ctx)
+	if _, err := os.Stat(target); errors.Is(err, os.ErrNotExist) {
+		logger.Warnf("Creating a new project config file at %q because the file does not already exist", target)
+		err = Save(target, Config{})
+		if err != nil {
+			return Config{}, err
+		}
+	}
+
 	log.FromContext(ctx).Tracef("Loading config from %s", target)
 	return loadFile(target)
 }

common/projectconfig/testdata/go/echo/echo.go

@@ -0,0 +1,21 @@
+package echo
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/TBD54566975/ftl/go-runtime/ftl" // Import the FTL SDK.
+)
+
+type EchoRequest struct {
+	Name ftl.Option[string] `json:"name"`
+}
+
+type EchoResponse struct {
+	Message string `json:"message"`
+}
+
+//ftl:verb
+func Echo(ctx context.Context, req EchoRequest) (EchoResponse, error) {
+	return EchoResponse{Message: fmt.Sprintf("Hello, %s!", req.Name.Default("anonymous"))}, nil
+}

common/projectconfig/testdata/go/echo/ftl.toml

@@ -0,0 +1,2 @@
+module = "echo"
+language = "go"

common/projectconfig/testdata/go/echo/go.mod

@@ -0,0 +1,45 @@
+module ftl/echo
+
+go 1.22.2
+
+toolchain go1.22.3
+
+require github.com/TBD54566975/ftl v0.234.0
+
+require (
+	connectrpc.com/connect v1.16.1 // indirect
+	connectrpc.com/grpcreflect v1.2.0 // indirect
+	connectrpc.com/otelconnect v0.7.0 // indirect
+	github.com/alecthomas/concurrency v0.0.2 // indirect
+	github.com/alecthomas/participle/v2 v2.1.1 // indirect
+	github.com/alecthomas/types v0.16.0 // indirect
+	github.com/alessio/shellescape v1.4.2 // indirect
+	github.com/danieljoos/wincred v1.2.0 // indirect
+	github.com/go-logr/logr v1.4.2 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/godbus/dbus/v5 v5.1.0 // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20221227161230-091c0ba34f0a // indirect
+	github.com/jackc/pgx/v5 v5.6.0 // indirect
+	github.com/jackc/puddle/v2 v2.2.1 // indirect
+	github.com/jpillora/backoff v1.0.0 // indirect
+	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/multiformats/go-base36 v0.2.0 // indirect
+	github.com/swaggest/jsonschema-go v0.3.70 // indirect
+	github.com/swaggest/refl v1.3.0 // indirect
+	github.com/zalando/go-keyring v0.2.4 // indirect
+	go.opentelemetry.io/otel v1.27.0 // indirect
+	go.opentelemetry.io/otel/metric v1.27.0 // indirect
+	go.opentelemetry.io/otel/trace v1.27.0 // indirect
+	golang.org/x/crypto v0.23.0 // indirect
+	golang.org/x/exp v0.0.0-20240506185415-9bf2ced13842 // indirect
+	golang.org/x/mod v0.17.0 // indirect
+	golang.org/x/net v0.25.0 // indirect
+	golang.org/x/sync v0.7.0 // indirect
+	golang.org/x/sys v0.20.0 // indirect
+	golang.org/x/text v0.15.0 // indirect
+	google.golang.org/protobuf v1.34.1 // indirect
+)
+
+replace github.com/TBD54566975/ftl => ../../../../..