Code cleaning and refactoring - Phase 1

[VisargD]

As a part of phase 1, the following changes can be done to clean up the code so that redundant code can be replaced by single functions and to make provider addition easier:

• Standardize all the provider API configs to have same functions: headers, getBaseURL and getEndpoint.
• Remove all the provider specific if/else-if blocks from tryPost and tryPostProxy by making use of the above mentioned standardized functions.
• Create error response builder function and reuse it for all the providers to keep control in one place instead of having it in all providers separately.
• Create a VALID_PROVIDERS array which can be used in requestValidator middleware instead of import all the providers names separately