Be more specific about what certificate format is expected in NodeMetadataPayload

[fjarri]

This is a relaxed version of #12 (which will be archived). Here we only changed NodeMetadataPayload.certificate_bytes to certificate_der to provide a hint on what certificate format is expected. Since certificates are not currently used on Rust side, we can live without deserialization for a while.

See comments in #12 about the compatibility problems between Python- and Rust-created certificates. Perhaps it will be somehow fixed in the future, or we will actually need certificate objects on Rust side and will have to deal with the differences.

A compromise would be to just deserialize the certificate on the Rust side, but not expose it - it can be added later, since it won't break backward compatibility. It just feels silly to do all the deserialization work and then not use the resulting object. According to my measurements, doing that increases deserialization time from 68us to 80us - quite noticeable on a relative scale (although it still means that a 1000 nodes, a typical size of our network, are deserialized in under 0.1s, which is negligible compared on the learning loop scale).

[codecov-commenter]

Codecov Report

Merging #13 (2c67ec6) into master (82702fc) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #13   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          12      12           
  Lines         423     423           
======================================
  Misses        423     423           

Impacted Files	Coverage Δ
nucypher-core/src/node_metadata.rs	0.00% <ø> (ø)

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