Support ASN.1 metadata

[joshuagl]

(NOTE: this issue copied from docs/ROADMAP.md)

Description of issue or feature request:

We currently support only a JSON-compatible metadata format.

Current behavior:

Only JSON-compatible metadata is supported

Expected behavior:

ASN.1 metadata files are supported including decoding, encoding, signing over, verifying signatures over

It may be worth considering making the metadata format abstract enough that integrators could easily implement their own metadata format, in a similar fashion to the StorageBackendInterface support.

[joshuagl]

Some preliminary work to define metadata in ASN.1 formats exists in #779

[joshuagl]

Adding the "refactor" milestone, as the notion of abstracting away the metadata format should be considered as part of any refactoring effort.

[trishankatdatadog]

I did a lot of work in this space once. Let me know if you want to collaborate on a POUF.

[lukpueh]

• related: New API: Don't hardcode metadata files extension #1333