FINERACT-2311: Add Buy Down Fees transaction support

[alberto-art3ch]

Description

As part of creating a Buy Down Fee transaction, the template call for this transaction should retrieve the maximum allowed amount:

Formula:
Maximum allowed amount = Approved amount of the loan - total disbursed amount - total buy down fees - total capitalized income

FINERACT-2311

Checklist

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[adamsaghy]

@alberto-art3ch

  Test givenMissingEventConfigurationWhenValidatedThenThrowException() FAILED

  org.opentest4j.AssertionFailedError: expected: <true> but was: <false>
      at app//org.apache.fineract.infrastructure.event.external.service.ExternalEventConfigurationValidationServiceTest.givenMissingEventConfigurationWhenValidatedThenThrowException(ExternalEventConfigurationValidationServiceTest.java:221)

[adamsaghy]

@alberto-art3ch Please rebase

[adamsaghy]

This PR looks much more than "adding Buy Down fee template API"

[adamsaghy]

This PR looks way bigger than simply adding template API for Buy down transaction

[adamsaghy]

LGTM