feat: add v5-custom-fields resource type support

[shsketch]

Summary

• Add CustomField struct for shared custom field item representation (used in both metadata and secret data)
• Add SecretDataTypeV5CustomFields and ResourceMetadataTypeV5CustomFields types matching the Passbolt v5.3+ schema
• Add fallback JSON schema for v5-custom-fields in ResourceSchemas
• Add v5-custom-fields case in GetResourceFromDataWithOptions to decrypt metadata and extract password from the first custom field of type "password"

Context

Passbolt v5.3+ introduced the v5-custom-fields resource type (slug: v5-custom-fields). Without this patch, any resource using this type causes GetResource / GetResourceFromData to return ErrUnsupportedResourceType.

Test plan

• Verify v5-custom-fields resources can be retrieved via GetResource
• Verify metadata fields (name, uris, description, custom_fields) are correctly parsed
• Verify password extraction from secret custom fields works
• Verify existing resource types are unaffected

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[cedricherzog-passbolt]

Hi, thanks for taking the time to contribute!

We really appreciate it. We're currently working on a broader fix for this in #67, which should address this issue as part of a more comprehensive solution.

With that in mind, I'll be closing this PR for now, but please don't hesitate to get involved in #67 if you'd like to help shape the direction there.