Refactor environment variables to configuration and registration (#90)

Close #21.

Refactor environment variables to configuration file (config.yaml) and registration file (.runner).

The old environment variables are still supported, but warning logs will be printed.

Like:

```text
$ GITEA_DEBUG=true ./act_runner -c config.yaml daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG has been ignored because config file is used

$ GITEA_DEBUG=true ./act_runner daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG will be deprecated, please use config file instead
```

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/90
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>

config/registration.go

@@ -0,0 +1,54 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package config
+
+import (
+	"encoding/json"
+	"os"
+)
+
+const registrationWarning = "This file is automatically generated by act-runner. Do not edit it manually unless you know what you are doing. Removing this file will cause act runner to re-register as a new runner."
+
+// Registration is the registration information for a runner
+type Registration struct {
+	Warning string `json:"WARNING"` // Warning message to display, it's always the registrationWarning constant
+
+	ID      int64    `json:"id"`
+	UUID    string   `json:"uuid"`
+	Name    string   `json:"name"`
+	Token   string   `json:"token"`
+	Address string   `json:"address"`
+	Labels  []string `json:"labels"`
+}
+
+func LoadRegistration(file string) (*Registration, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	var reg Registration
+	if err := json.NewDecoder(f).Decode(&reg); err != nil {
+		return nil, err
+	}
+
+	reg.Warning = ""
+
+	return &reg, nil
+}
+
+func SaveRegistration(file string, reg *Registration) error {
+	f, err := os.Create(file)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	reg.Warning = registrationWarning
+
+	enc := json.NewEncoder(f)
+	enc.SetIndent("", "  ")
+	return enc.Encode(reg)
+}