Add proof-measure client (public securedBy / measured-immutability service)

proof-measure is a separate, public, unauthenticated Dragonchain service. Adds:
- client.UnauthenticatedClient: HMAC-free transport mirroring Client's
  marshal/decode/error handling.
- proofmeasure.ProofMeasureClient: GetSecurity / Report / Health, default base
  URL https://proof-measure.dragonchain.com; standalone + a DragonchainSDK.ProofMeasure handle.
- models for SecurityResult/RawMeasure/ReportRequest/TransactionReport/etc
  (decimals as strings, timestamps int64).
- httptest unit tests.

dragonchain.go

@@ -55,6 +55,7 @@ import (
 	"git.dragonchain.com/dragonchain/prime-sdk-go/block"
 	"git.dragonchain.com/dragonchain/prime-sdk-go/client"
 	"git.dragonchain.com/dragonchain/prime-sdk-go/contract"
+	"git.dragonchain.com/dragonchain/prime-sdk-go/proofmeasure"
 	"git.dragonchain.com/dragonchain/prime-sdk-go/system"
 	"git.dragonchain.com/dragonchain/prime-sdk-go/transaction"
 	"git.dragonchain.com/dragonchain/prime-sdk-go/transactiontype"
@@ -69,6 +70,11 @@ type DragonchainSDK struct {
 	Contract        *contract.ContractClient
 	Block           *block.BlockClient
 	System          *system.SystemClient
+	// ProofMeasure calls the public proof-measure service (measured immutability
+	// / "securedBy"). It is a separate, unauthenticated service, so this handle
+	// targets its default public endpoint (proofmeasure.DefaultBaseURL); for a
+	// custom endpoint construct a proofmeasure.ProofMeasureClient directly.
+	ProofMeasure *proofmeasure.ProofMeasureClient
 }
 
 // NewDragonchainSDK creates a new Dragonchain SDK client.
@@ -98,6 +104,7 @@ func NewDragonchainSDKWithHTTPClient(publicID, authKeyID, authKey, baseURL strin
 		Contract:        contract.NewContractClient(c),
 		Block:           block.NewBlockClient(c),
 		System:          system.NewSystemClient(c),
+		ProofMeasure:    proofmeasure.NewProofMeasureClientWithHTTPClient("", hc),
 	}
 }