[Feature Request] Allow multiple annotate.md files

Nobody has claimed this yet.

Currently it is only possible to upload one file per step for a fixed style i think.

It would be nice if you could define multiple annotate files, with their own style and context.
e.g.
```
- label: "SyTest - :go: 1.13 / :postgres: 9.6"
agents:
queue: "medium"
env:
POSTGRES: "1"
command:
- "bash build.sh"
- "bash /bootstrap.sh dendrite"
plugins:
- docker#v3.0.1:
image: "matrixdotorg/sytest-dendrite"
propagate-environment: true
always-pull: true
workdir: "/src"
entrypoint: '/bin/sh'
shell: ["-x", "-c"]
init: false
debug: true
mount-buildkite-agent: false
volumes: ["./logs:/logs"]
- artifacts#v1.2.0:
upload: [ "logs/**/*.log", "logs/**/*.log.*", "logs/results.tap" ]
- matrix-org/annotate:
- path: "logs/annotate_errors.md"
style: "error"
context: "annotate_errors"
- path: "logs/annotate_info.md"
style: "info"
context: "annotate_info"
- path: "logs/are-we-synapse-yet.md"
style: "default"
context: "are-we-synapse-yet"
```

Start at the existing matrix-org/annotate configuration that currently accepts one annotate.md file per step. Compare it with the requested entries for annotate_errors.md, annotate_info.md, and are-we-synapse-yet.md, including their style and context values. Done means multiple files can be configured independently without losing the current single-file behavior.

Written by the indexing model from the issue text.