[feat] Add header authentication support

In some cases there are needs to authenticate user not in Taskcafe itself.
For this reason option server.remote_user_header was added.

```toml
[server]
remote_user_header = "X-Remote-User"
```

With turned on Taskcafe listens X-Remote-User http header and skip
password checking. But still check user existence and activity flag.

internal/utils/security.go

@@ -9,6 +9,11 @@ import (
 type SecurityConfig struct {
 	AccessTokenExpiration time.Duration
 	Secret                []byte
+	UserAuthHeader        string
+}
+
+func (c SecurityConfig) IsRemoteAuth() bool {
+	return c.UserAuthHeader != ""
 }
 
 func GetSecurityConfig(accessTokenExp string, secret []byte) (SecurityConfig, error) {