feat(rust/signed-doc): check protected metadata field support

[no30bit]

Description

Implements reporting of unsupported fields found in a protected header. Fields are deemed unsupported if existing logic discarded them by not deserialising into corresponding Metadata fields (if any).

I find mismatch between the number of protected fields and the number of fields that end up in Metadata. To this end there's new counting functions.

I prefer this approach over validating each key individually against a list of supported keys, because the later doesn't by itself solve duplication potential and the later is always more intense computationally.

Related Issue(s)

Closes #316

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• My code follows the style guidelines of this project
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• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
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• I have added tests that prove my fix is effective or that my feature works
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[no30bit]

I'll add the unit tests if the implementation is fine with everyone

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 319/319}$ | ${\color{red}Fail: 0/319}$ |

[Mr-Leshiy]

Not agree with the naming and the description for this function.
if I am not mistaken, but this function returns the number of all header fields no ?
If so you should also take into account other header fields like alg, kid etc.

[no30bit]

this function returns the number of all header fields

Not exactly. As I mentioned in the doc comment, the function excludes those that are not used when deserialising.

The naming idea is that alg and kid aren't related to metadata and I focus only on metadata here. I draw conclusion that they aren't, because they don't end up in the Metadata type, thus irrelevant at this level.

This is intentional, because my implementation depends on the structure of the Metadata type. If I counted alg and kid, I would get N+2 while InnerMetadata::from_protected_header doesn't use those fields and our InnerMetadata doesn't contain them (by N I mean the number of InnerMetadata fields in a protected header). Then when I count the number of fields that are successfully deserialised (M, where M ≤ N), I would always have a failing case M ≤ N < N + 2, because the invariant is N = M.

[no30bit]

Now, the validity of kid and alg are guaranteed by coset implementation of <coset::Header as AsCborValue>::from_cbor_value. There these reserved fields are explicitly matched and converted.

Any fields that aren't reserved are put into the Header::rest field. Our metadata fields except for the content_type all lie in there. So this is the only place to check for dups. But I account for the content_type field by adding it to the relevant count, as it is a part of InnerMetadata but not a part of the Header::rest.

[no30bit]

Abandoned and superseded by #334 and #332 solutions