Sliding Sync: Return room tags in account data extension

[MadLittleMods]

Return room tags in account data extension

The account data extension was also updated to avoid copies when we pull the data out of the cache.

Fix #17694

Dev notes

Sync v2 incremental sync response with some rooms tags in the account data (favouriting a room)

{
    "next_batch": "s5241945698_757284974_2782863_3110944073_3359903529_260642965_1380548560_11007322260_0_338462",
    "device_one_time_keys_count": {
        "signed_curve25519": 50
    },
    "org.matrix.msc2732.device_unused_fallback_key_types": [
        "signed_curve25519"
    ],
    "device_unused_fallback_key_types": [
        "signed_curve25519"
    ],
    "rooms": {
        "join": {
            "!xxx:matrix.org": {
                "timeline": {
                    "events": [],
                    "prev_batch": "s5241945695_757284974_2782854_3110944063_3359903514_260642965_1380548559_11007322258_0_338462",
                    "limited": false
                },
                "state": {
                    "events": []
                },
                "account_data": {
                    "events": [
                        {
                            "type": "m.tag",
                            "content": {
                                "tags": {
                                    "m.favourite": {}
                                }
                            }
                        }
                    ]
                },
                "ephemeral": {
                    "events": []
                },
                "unread_notifications": {
                    "notification_count": 0,
                    "highlight_count": 0
                },
                "summary": {}
            }
        }
    }
}

ChainMap

DeepChainMap example in the Python docs: https://docs.python.org/3/library/collections.html

aka NestedChainMap

Pull Request Checklist

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[MadLittleMods]

Thanks for the review @devonh 🐏