Consider adding full EC2 context

[chuwy]

With #2 we can add custom context with instance identity document (/latest/dynamic/instance-identity/document/) for EC2. Also we could add context based on full instance metadata (/latest/meta-data/). To do that we need to:

• write parser
• transform instances to leave only meaningful data
• create schema for transformed schema

[chuwy]

Actually, since parser is written user can add this context by himself, but since no schema provided and we still don't know how it will be handled, I think we should leave it as undocumented feature.

[chuwy]

Pushing back to "Far-out stuff". Blocked by snowplow-archive/schema-guru#111