Add PKI secrets engine management for issuers, keys, and roles

[SergeyFilippov]

Extends the PKI secrets engine support with full CRUD management for issuers, keys, and roles, and drops net7.0 from the target frameworks.

• Added IPKIIssuers interface and PKISecretIssuersProvider with operations to list, read, update, and delete PKI issuers, including issuer config management
• Added IPKIKeys interface and PKISecretKeysProvider with operations to list, read, import, generate, and delete PKI keys
• Added IPKIRoles interface and PKISecretRolesProvider with operations to list, read, create, update, and delete PKI roles
• Added supporting data models: IssuerData, IssuerConfigRequestData, IssuerConfigResponseData, KeyData, RoleData, and related enums (IssuerFormat, IssuerPrivetKeyFormat, KeyGenerationType, PrivateKeyType, KeyUsageConstants, ExtendedKeyUsageConstants)
• Exposed Issuers, Keys, and Roles as top-level properties on IPKISecretsEngine
• Removed net7.0 from the list of target frameworks in VaultSharp.csproj