Add tags to SafeApp

[fmrsabino]

Related with safe-global/safe-config-service#486 and 5afe/safe-react#3698

• Enables serialisation and deserialisation of tags for a SafeApp
• Serialisation is enabled IFF SAFE_APPS_TAGS_FEATURE_ENABLED is set to true

GET /v1/chains/<chain_id>/safe-apps

{
    "id": 1,
    "name": "OpenZeppelin",
    "tags": [
	    "tag1",
	    "tag2",
	    "tag3"
    ]
    ...
}

[coveralls]

Pull Request Test Coverage Report for Build 2160682873

• 43 of 45 (95.56%) changed or added relevant lines in 6 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 88.671%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/common/models/backend/safe_apps.rs	0	1	0.0%
src/routes/safe_apps/tests/routes.rs	31	32	96.88%

Files with Coverage Reduction	New Missed Lines	%
src/routes/safe_apps/converters.rs	1	58.57%
src/common/converters/tests/safe_app.rs	5	60.76%

Totals
Change from base Build 2102745780:	0.02%
Covered Lines:	11881
Relevant Lines:	13399

---

💛 - Coveralls