pkg/cover: export aggregated coverage

[tarasmadan]

Is your feature request related to a problem? Please describe.
External services need access to the aggregated coverage in some machine readable format.
We know people are parsing our html coverage file.

Describe the solution you'd like
Json will work here.

It may be the /graph/coverage/file?...&json=1.
The format TBD.

Additional context
We don't have the reliable source code provider yet.
Assuming we need some machine readable data, the result w/o source code looks acceptable.

The requester needs access to the coverage and associated syz-lang programs.
The value of the coverage w/o programs is a question.
The complexity of the coverage export is low now. The complexity of the programs export is high (but is doable once prioritized).