Refactor ArtifactSignature to use asHex method to return signature

 -- Remove SignatureFormatter functional interface
 -- Update SignerForIdentifier

signing/src/main/java/tech/pegasys/web3signer/signing/ArtifactSignature.java

@@ -12,7 +12,7 @@
  */
 package tech.pegasys.web3signer.signing;
 
-public interface ArtifactSignature {
+public interface ArtifactSignature<T> {
 
   /**
    * Returns the type of key used to sign the artifact
@@ -27,4 +27,11 @@ public interface ArtifactSignature {
    * @return the signature data in hex format
    */
   String asHex();
+
+  /**
+   * Returns the signature data
+   *
+   * @return the signature data
+   */
+  T getSignatureData();
 }

signing/src/main/java/tech/pegasys/web3signer/signing/ArtifactSignerProvider.java

@@ -48,9 +48,7 @@ public interface ArtifactSignerProvider extends Closeable {
    * @param consensusPubKey the identifier of the consensus signer
    * @return Map of Key Type (BLS, SECP256K1) and corresponding proxy identifiers
    */
-  default Map<KeyType, Set<String>> getProxyIdentifiers(final String consensusPubKey) {
-    throw new UnsupportedOperationException("Proxy signers are not supported by this provider");
-  }
+  Map<KeyType, Set<String>> getProxyIdentifiers(final String consensusPubKey);
 
   /**
    * Add a new signer to the signer provider.

signing/src/main/java/tech/pegasys/web3signer/signing/BlsArtifactSignature.java

@@ -14,7 +14,7 @@
 
 import tech.pegasys.teku.bls.BLSSignature;
 
-public class BlsArtifactSignature implements ArtifactSignature {
+public class BlsArtifactSignature implements ArtifactSignature<BLSSignature> {
   private final BLSSignature blsSignature;
 
   public BlsArtifactSignature(final BLSSignature blsSignature) {
@@ -26,6 +26,7 @@ public KeyType getType() {
     return KeyType.BLS;
   }
 
+  @Override
   public BLSSignature getSignatureData() {
     return blsSignature;
   }

signing/src/main/java/tech/pegasys/web3signer/signing/SecpArtifactSignature.java

@@ -19,7 +19,7 @@
 import org.apache.tuweni.bytes.Bytes32;
 import org.web3j.utils.Numeric;
 
-public class SecpArtifactSignature implements ArtifactSignature {
+public class SecpArtifactSignature implements ArtifactSignature<Signature> {
   private final Signature signature;
 
   public SecpArtifactSignature(final Signature signature) {
@@ -31,6 +31,7 @@ public KeyType getType() {
     return KeyType.SECP256K1;
   }
 
+  @Override
   public Signature getSignatureData() {
     return signature;
   }

signing/src/main/java/tech/pegasys/web3signer/signing/config/SecpArtifactSignerProviderAdapter.java

@@ -17,6 +17,7 @@
 
 import tech.pegasys.web3signer.signing.ArtifactSigner;
 import tech.pegasys.web3signer.signing.ArtifactSignerProvider;
+import tech.pegasys.web3signer.signing.KeyType;
 
 import java.util.HashMap;
 import java.util.Map;