Add plaintext support for NanoTDF within the SDK

[jrschumacher]

The current implementation of nanoTDF in the SDK only supports encrypted policy. The nanoTDF spec explicitly supports encrypted and plaintext policy.

The current need of plaintext policy is to both be compatible with Base TDF (ZTDF), but also to enable reading the policy for visibility trimming without the need to over-privledge service accounts.

See spec: https://github.com/opentdf/spec/blob/main/schema/nanotdf/README.md

Policy mode hardcoded as encrypted: https://github.com/opentdf/platform/blob/main/sdk/nanotdf.go#L502-L503

Policy body encrypted:

platform/sdk/nanotdf.go

Lines 547 to 554 in ee67026

	const (
	kIvLength = 12
	)
	iv := make([]byte, kIvLength)
	cipherText, err := aesGcm.EncryptWithIVAndTagSize(iv, policyObjectAsStr, tagSize)
	if err != nil {
	return nil, 0, 0, fmt.Errorf("AesGcm.EncryptWithIVAndTagSize failed:%w", err)
	}