Refactor command records and simplify document handling

- Updated `ReceiverGetOrCreateCommand` to include a required `IEnumerable<Signature>`.
- Modified `DocumentCreateCommand` to remove `DataAsByte` and enforce a required `DataAsBase64` property with immutability.
- Cleaned up SQL command formatting in `EnvelopeReceiverAddReadSQL`.
- Simplified document validation logic in `EnvelopeReceiverController` by removing redundant checks.
- Changed hardcoded connection string to a variable `_cnnStr` for improved security and flexibility.

EnvelopeGenerator.Application/SQL/EnvelopeReceiverAddReadSQL.cs

@@ -13,15 +13,14 @@ public class EnvelopeReceiverAddReadSQL : ISQL<Envelope>
     /// 
     /// </summary>
     public string Raw => @"
-		USE [DD_ECM]
 		DECLARE	@OUT_RECEIVER_ID int
 
 		EXEC	[dbo].[PRSIG_API_CREATE_RECEIVER]
-				@ENV_UID = @ENV_UID,
-				@EMAIL_ADRESS = @EMAIL_ADRESS,
-				@SALUTATION = @SALUTATION,
-				@PHONE = @PHONE,
-				@OUT_RECEIVER_ID = @OUT_RECEIVER_ID OUTPUT
+				@ENV_UID,
+				@EMAIL_ADRESS,
+				@SALUTATION,
+				@PHONE,
+				@OUT_RECEIVER_ID OUTPUT
 
 		SELECT TOP(1) *
 		FROM TBSIG_ENVELOPE_RECEIVER