bridgev2: Support for caching and storage of Portal/Ghost related metadata

[purpshell]

Current Problem

Pretext

One of the features of the Twitter platform (supported in the Twitter bridge at mautrix/twitter) is read receipts. The way this feature works is that on every long poll, it will return the last message id the other side read.

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Unfortunately, due to a lack of means of storage for this variable (memory caching or db storage, or a hybrid solution), we are unable to compare the new read id with the previous state, proving it nigh impossible to implement in a memory-safe, functional, and efficient manner.

Proposed Solution

The addition of metadata-related fields for Portals and Ghosts, this way, it will be possible to set chat-specific or participant-specific parameters, that are automatically cached and manged by the framework.