Extend Native AOT support for PKI management APIs

[Copilot]

The upstream PKI management commit (rajanadar/VaultSharp@8cb298c) added Issuers/Keys/Roles APIs whose types were not covered by the existing Native AOT source-generation context, and whose enum converters used the reflection-based non-generic JsonStringEnumConverter.

Enum converters

Four new PKI enums updated to use the AOT-safe generic form under NET8_0_OR_GREATER, preserving the non-generic fallback for older TFMs — matching the pattern already established in the PR for CertificateFormat, PrivateKeyFormat, etc.:

#if NET8_0_OR_GREATER
    [JsonConverter(typeof(JsonStringEnumConverter<IssuerFormat>))]
#else
    [JsonConverter(typeof(JsonStringEnumConverter))]
#endif
public enum IssuerFormat { ... }

Affected: IssuerFormat, IssuerPrivateKeyFormat, PrivateKeyType, PrivateKeyRoleType.

VaultSharpJsonContext registrations

Added [JsonSerializable] entries for all new PKI request/response types:

• Issuers: Secret<IssuerResponseData>, Secret<IssuerConfigResponseData>, Secret<IssuerData>, Secret<KeysData>, plus bare DTO types IssuerData, IssuerConfigRequestData, IssuerConfigResponseData, IssuerResponseData
• Keys: Secret<KeysData>, Secret<KeyData>, Secret<ImportKeyData>, plus bare KeyData, ImportKeyData
• Roles: Secret<RolesData>, Secret<RoleData>, plus bare RoleData, RolesData
• Supporting: Dictionary<string, KeyInfo> variants for both namespaces

KeysData and KeyInfo each exist in both Issuers and Keys namespaces; all four are disambiguated with TypeInfoPropertyName to avoid source-gen naming conflicts.

File rename

IssuerPrivetKeyFormat.cs → IssuerPrivateKeyFormat.cs (typo; type name IssuerPrivateKeyFormat was already correct).