FINERACT-2169: Apply dto for external event config api;

[VictorPavfurious]

Description

1. Replace string req/res to dto;
2. Update swagger docs and clean up;

Checklist

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• Follow coding conventions at https://cwiki.apache.org/confluence/display/FINERACT/Coding+Conventions.

• Add required Swagger annotation and update API documentation at fineract-provider/src/main/resources/static/legacy-docs/apiLive.htm with details of any API changes

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[VictorPavfurious]

@adamsaghy , This is pr had success completed tests, but i have to change commit, because of i have started using my deprecated email here.. So, i going to fix it and then you can merge OR run again all checks.
Sorry for my mistake

[VictorPavfurious]

Fixed commit.