Cross platform test and UTF-8 fixes

[jku]

This should fix #1552

• added a test workflow based on idea taken from model-transparency
• the test immediately fails because apparently we write cp1252 on Windows (and this is different with rekor 2 checkpoints that contain an mdash)
• I've fixed the issues by being explicit about encoding: This changes serialization on Windows. Maybe we should actually be reading in bytes but I wanted the change to be minimal here.

Some details on the impact of this issue:

• It is possible that some bundles produced on older releases on windows are no longer verifiable on windows when using a newer release (if the bundles did have encoding issues they used to be verifiable on windows but were never verifiable on other platforms)
• In practice it seems this issue triggers specifically with rekor v2 because of the new checkpoint format

[spencerschrock]

This should fix #1552

I noticed all the encoding changes are in _cli.py. Is this something we (model-transparency) would be hitting in our call path? Or is sigstore-python CLI used by rekor2, which we may be hitting by upgrading to sigstore-python v4?

[jku]

I noticed all the encoding changes are in _cli.py. Is this something we (model-transparency) would be hitting in our call path? Or is sigstore-python CLI used by rekor2, which we may be hitting by upgrading to sigstore-python v4?

oh good point: _cli is not used by other modules so it's possible that some of your code has the same issues (maybe it was made following the _cli module here?)

[spencerschrock]

oh good point: _cli is not used by other modules so it's possible that some of your code has the same issues

Indeed, we were also reading and writing signatures without specifying an encoding. Forcing everything to utf-8 seemed to resolve our issues

Maybe we should actually be reading in bytes

model-transparency will be thinking about this too

[spencerschrock]

Maybe we should actually be reading in bytes

For the JSON bundle, I think UTF-8 is more conformant depending on how one interprets "closed ecosystem"?

https://datatracker.ietf.org/doc/html/rfc8259#section-8.1

JSON text exchanged between systems that are not part of a closed
ecosystem MUST be encoded using UTF-8 [RFC3629].

[jku]

To be clear, I didn't mean we should think about changing the representation on disk: utf-8 text is fine --I was just thinking that it might be clearer to read bytes and pass those to the json parser (double checking that it actually uses utf-8 in all cases).