Add snapshots endpoint

[zhouzhuojie]

Adding snapshots endpoint. This is to prepare the audit trail for "who edit what" in UI

[zhouzhuojie]

tested with curl

curl --request GET \
  --url http://localhost:18000/api/v1/flags/2/snapshots

[
	{
		"flag": {
			"dataRecordsEnabled": false,
			"description": "something new",
			"enabled": false,
			"id": 2,
			"segments": [
				{
					"constraints": [],
					"description": "123",
					"distributions": [],
					"id": 4,
					"rank": 0,
					"rolloutPercent": 50
				}
			],
			"variants": []
		},
		"id": 31
	},
	{
		"flag": {
			"dataRecordsEnabled": false,
			"description": "something new",
			"enabled": false,
			"id": 2,
			"segments": [],
			"variants": []
		},
		"id": 30
	},
	{
		"flag": {
			"dataRecordsEnabled": false,
			"description": "something new",
			"enabled": false,
			"id": 2,
			"segments": [
				{
					"constraints": [],
					"description": "123",
					"distributions": [],
					"id": 3,
					"rank": 0,
					"rolloutPercent": 50
				}
			],
			"variants": []
		},
		"id": 29
	},
	{
		"flag": {
			"dataRecordsEnabled": false,
			"description": "something new",
			"enabled": false,
			"id": 2,
			"segments": [],
			"variants": []
		},
		"id": 28
	}
]