Define com.github / com.gitlab and other metadata schemas in use by matrix-hookshot

[matrix-hookshot](https://github.com/Half-Shot/matrix-hookshot) bridges various things like GitHub, GitLab, jira etc. One of the key features is that it provides rich metadata in each bridged event, so that clients / bots can hook into that data and display it in a rich format if they prefer (think: extensible events).

One of the problems facing the project is that Matrix doesn't yet have a nice way to define *external* schemas like a GitHub Pull Request, because it doesn't fit into the bucket of a core matrix event type, but neither should it be excluded from the spec entirely (which would cause different implementations to implement it differently and under different keys).

This issue is here to ensure that once such a spec mechanism exists, Hookshot and other interested implementations should rapidly try to define their schemas within Matrix so that there is less divergence.

No files or tests are named. Start by reviewing the external-schema mechanism when it exists and the metadata emitted by matrix-hookshot, then define schemas for com.github, com.gitlab, and the other metadata described in the issue. Done means the schemas are specified within Matrix so implementations can use consistent keys.

Written by the indexing model from the issue text.