Make SignatureAndPayloadConfig and PolicyBindingConfig initializers public

## Issue

The `SignatureAndPayloadConfig` and `PolicyBindingConfig` structs have internal initializers, preventing external packages from using the signature functionality.

## Current Behavior

```swift
// In external package using OpenTDFKit
let config = SignatureAndPayloadConfig(
signed: true,
signatureCurve: .secp256r1,
payloadCipher: .aes256GCM128
)
// Error: 'SignatureAndPayloadConfig' initializer is inaccessible due to 'internal' protection level
```

The `addSignatureToNanoTDF()` function is public and documented, but cannot be used because the required `SignatureAndPayloadConfig` parameter cannot be constructed externally.

## Expected Behavior

Users should be able to create signed NanoTDFs from external packages:

```swift
import OpenTDFKit
import CryptoKit

var nanoTDF = try await createNanoTDF(...)

let config = SignatureAndPayloadConfig(
signed: true,
signatureCurve: .secp256r1,
payloadCipher: .aes256GCM128
)

let privateKey = P256.Signing.PrivateKey()
try await addSignatureToNanoTDF(
nanoTDF: &nanoTDF,
privateKey: privateKey,
config: config
)
```

## Proposed Solution

Add public initializers to both structs:

### SignatureAndPayloadConfig

```swift
public struct SignatureAndPayloadConfig: Sendable {
var signed: Bool
var signatureCurve: Curve?
let payloadCipher: Cipher?

// Add this public initializer
public init(signed: Bool, signatureCurve: Curve?, payloadCipher: Cipher?) {
self.signed = signed
self.signatureCurve = signatureCurve
self.payloadCipher = payloadCipher
}

// ... existing toData() method
}
```

### PolicyBindingConfig

```swift
public struct PolicyBindingConfig: Sendable {
var ecdsaBinding: Bool
var curve: Curve

// Add this public initializer
public init(ecdsaBinding: Bool, curve: Curve) {
self.ecdsaBinding = ecdsaBinding
self.curve = curve
}

// ... existing toData() method
}
```

## Impact

Without public initializers:
- ❌ Cannot use `addSignatureToNanoTDF()` from external packages
- ❌ Cannot manually construct NanoTDF headers with custom configs
- ❌ Signature functionality is effectively unusable outside OpenTDFKit itself

With public initializers:
- ✅ Full signature support for external packages
- ✅ Ability to create signed NanoTDFs for NTDF Profile v1.2 chains
- ✅ Custom header configurations for advanced use cases

## Use Case

We're implementing NTDF Profile v1.2 Chain of Trust authorization where each link should be signed:

1. Origin Link (PE attestation) - signed by user's DID key
2. Intermediate Link (NPE attestation) - signed by app's key
3. Terminal Link (from IdP) - signed by IdP's key

Currently blocked because we cannot create `SignatureAndPayloadConfig` to pass to `addSignatureToNanoTDF()`.

Related: arkavo-org/app#160, arkavo-org/app#161

## Files to Modify

`OpenTDFKit/NanoTDF.swift`:
- Lines 523-552: `SignatureAndPayloadConfig` struct
- Lines 502-521: `PolicyBindingConfig` struct

Just add public initializers to both structs (3 lines each).

## Workaround (Current)

Currently relying on GMAC policy binding instead of signatures, which works but is not ideal for multi-party chains where each party should cryptographically sign their attestation.

## Priority

Medium - Signature functionality exists but is inaccessible. GMAC binding provides security, but signatures would enable stronger attestation chains.

Open OpenTDFKit/NanoTDF.swift and inspect PolicyBindingConfig around lines 502-521 and SignatureAndPayloadConfig around lines 523-552. Make the two requested initializers usable from external packages, then verify that the construction examples in the issue compile and that the existing signature functionality remains usable.

Written by the indexing model from the issue text.