Updated to support Mambu 3.8 API changes

1. Added support for MBU-6661 (As a Developer, I need to update a single custom field).

1.1. Added support for HTTP PATCH method in RequestExecutorImpl
1.2. Enhanced  ApiDefinition and ServiceExecutor classes to support providing an optional “related entity” ID
1.3. Added wrapper support for new APIs to Update and Delete a single Custom Field for any of the following entities:
Clients and Groups (ClientService.java), LoanAccounts (LoanService),   SavingsAccounts (SavingsService), Branches and Centres (OrganizationService) and Users (UserService)
1.4. Added helper method in ServiceHelper
1.4 Updated corresponding DemoTest services. Updated DemoUtil to provide helper methods for DemoTest services
1.5.  Updated Maven’s pom file to use version 4.2.1 for httpClient jar (org.apache.httpcomponents).
Note: HTTP PATCH is available starting with the httpClient version 4.2

2. Added support for MBU-6620 and MBU-6621 (adding “Identifier” and “AccountName” as new configurable fields for Transaction channels)

2.1  Created new methods for account transactions accepting Transaction Channel fields. These new methods take TransactionDetails object as input parameter. Specifically:
LoanService:  disburseLoanAccount() and  makeLoanRepayment()
SavingsService: makeWithdrawal() and makeDeposit()
2.2. The original implementations of these methods are marked as @Deprecated and could be removed in future releases
2.3 Updated ServiceHelper to provide helper methods
2.4 Updated DemoTest services accordingly. Added helper methods in DemoUtil.

Misc:
- Marked as @Deprecated wrapper methods relying on default pagination parameters when a corresponding method with pagination parameters is available: getClients(boolean), getRapaymentsDueFromTo(), getLoanAccountRepayments()
- Refactored RequestExecutorImpl to use helper methods to process HTTP response (common to most HTTP methods)
- Updated README file

These enhancements are available in the 3.8 version of the build (Mambu-APIs-Java-3.8-bin.jar)
Maybe on the next refactoring you can use inheritance or the composition pattern to extract these updateCustomField and deleteCustomField methods to e.g. an abstract class so that they don't have to be implemented in almost every service.