Include Metadata Schema

[kairoaraujo]

What this PR does / why we need it

Include Metadata Schema for the DSSE envelopes.

Acceptance Criteria Met

• Docs changes if needed
• Testing changes if needed
• All workflow checks passing (automatically enforced)
• All review conversations resolved (automatically enforced)
• DCO Sign-off

Special notes for your reviewer:

[kairoaraujo]

Hey @jkjell,
Should we have a string as a value or any data structure?
Any data structure could bring some flexibility, such as in the unrecognized fields TUF uses.
IMO, it helps a lot in application integrations or even operational usage.

[jkjell]

❤️ the idea. The biggest unknown for me is if/how that complicates the creation of metadata (i.e. if we go the HTTP header route or something else)?

[kairoaraujo]

I suggest having it in the DSSE envelope JSON, but I know many people disagree.

{
    "payload": "payload.....",
    "payloadType": "https://witness.testifysec.com/...",
    "signatures": [
        {
            "keyid": "keyid...",
            "sig": "signature..."
        }
    ],
    "metadata": "any"
}

[codecov]

Codecov Report

Attention: Patch coverage is 0.41973% with 2847 lines in your changes are missing coverage. Please review.

Project coverage is 1.40%. Comparing base (a035c62) to head (05eac30).
Report is 81 commits behind head on main.

Files	Patch %	Lines
generated.go	0.00%	634 Missing ⚠️
ent/metadata_query.go	0.00%	383 Missing ⚠️
ent/mutation.go	0.00%	285 Missing ⚠️
ent/metadata_update.go	0.00%	250 Missing ⚠️
ent/gql_pagination.go	0.00%	193 Missing ⚠️
ent/gql_where_input.go	0.00%	148 Missing ⚠️
ent/metadata_create.go	0.00%	130 Missing ⚠️
ent/gql_collection.go	0.00%	119 Missing ⚠️
ent/dsse_update.go	0.00%	112 Missing ⚠️
ent/client.go	0.00%	98 Missing ⚠️
... and 16 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #269       +/-   ##
==========================================
- Coverage   82.40%   1.40%   -81.00%     
==========================================
  Files          10     125      +115     
  Lines         358   31290    +30932     
==========================================
+ Hits          295     441      +146     
- Misses         43   30792    +30749     
- Partials       20      57       +37     

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