FINERACT-2814: Buy down fee amortization strategy loan product attribute config

[oleksii-novikov-onix]

Description

Depends on apache/fineract#6439. Until it's merged, this PR includes that commit too.

1. Adds the BUY_DOWN_FEE_AMORTIZATION_STRATEGY loan product attribute with values DEFERRED or IMMEDIATE.
2. Creating an attribute now clears the attribute cache, as update and delete already do.
3. Adds e2e tests.

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