feat: reaction metadata

[varunsrin]

What is the feature you would like to implement?
Add a special metadata field to reactions that allow them to be interpreted in a different way (e.g. interpret as GM instead of Heart)

Why is this feature important?
Allows developers to build many new cast interactions on top of the existing action system by adding a minimal amount of metadata.

Will the protocol spec need to be updated??
Will require changes to Actions in Section 4

How should this feature be built? (optional)
TBD

Additional context
There is some overlap with #119 since polls could be composed as a form of metadata on top of a reaction, so consider how these might be implemented together.

[pfletcherhill]

How would this relate to the type field already on a ReactionAdd message? Currently the type is only like.

[pfletcherhill]

Proposal

Update the ReactionAdd message to include two (character limited) strings:

type ReactionBody = {
  cast: CastURL;
  type: string; // limit to 12 characters (chosen arbitrarily)
  meta: string; // limit to 25 characters (same as poll options eventually)
}

To save space, we could also limit the type to be a number like MessageType.

[varunsrin]

suggestions:

1. For type, since it is strictly typed, can we just use an enum instead of a string? If we only have 4 values, that will save a large amount of space, especially given that reactions are the most common action by volume,

2. For meta, i think we should keep it fairly small, maybe 4 bytes or so? People can use the delegate app identifiers in combination with the meta to determine how it should be interpreted.

[varunsrin]

moving to v2.1.0 per our discussion