Fix base64 format for JSON

[kentakayama]

EAT basically uses base64-url-text without '=' padding, except for the nonce-type.
However, Go's encoding/json package encodes binary data using standard base64 with padding by default.

This PR resolves that mismatch by ensuring base64url encoding without padding is used where required.

As part of this change, the swid.HashEntry has been replaced with Digest.
The former encodes a hash like [swid.Sha256, h'0000...'] as a string "sha256;AA...=".
In contrast, the new Digest type encodes it as an array [1,"AA..."] where "AA..." is base64url-encoded without padding.

This aligns with the following specifications:

• RFC 9711

binary-data = JC< base64-url-text, bstr>
base64-url-text = tstr .regexp "[A-Za-z0-9_-]+"

ueid-type = JC<base64-url-text .size (10..44) , bstr .size (7..33)>
hardware-model-type = JC<base64-url-text .size (4..44),
                         bytes .size (1..32)>
$$Claims-Set-Claims //=  (boot-seed-label => binary-data)

• draft-ietf-rats-eat-measured-component

bytes-b64u = text .b64u bytes
bytes8 = bytes .size 8
bytes8-b64u = text .b64u bytes8

digest = [
  alg: (int / text)
  val: digest-value-type
]

digest-value-type = eat.JC<bytes-b64u, bytes>

[kentakayama]

This PR should be ready for review. @thomas-fossati , could you please take a look when you have time?

[thomas-fossati]

One quick check: AFAIR, there is no special encoding for coswid.$version-scheme in the JSON serialisation. I.e., instead of ”multipartnumeric" it should be 1, no?

[kentakayama]

Exactly.
RFC 9393 states that CoSWID does not define any specific encoding or decoding rules for JSON.
https://datatracker.ietf.org/doc/html/rfc9393#name-the-concise-swid-tag-map

The string "multipartnumeric" comes from the veraison/swid implementation specification:
https://github.com/veraison/swid/blob/main/versionscheme.go#L40-L56

[thomas-fossati]

OK, thanks for double checking. We will need to fix it in veraison/swid then.

(EDIT: raised veraison/swid#47)

[thomas-fossati]

thanks @kentakayama !