[Accelerator 4] Adding Accelerator Gateway Implementation

[Ashi1993]

Adding Accelerator Gateway Implementation

This PR adds the Accelerator Gateway Implementation.

Issue link: https://github.com/wso2-enterprise/solutions-bfsi/issues/20

Doc Issue: Optional, link issue from documentation repository

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Development Checklist

1. Build complete solution with pull request in place.
2. Ran checkstyle plugin with pull request in place.
3. Ran Findbugs plugin with pull request in place.
4. Ran FindSecurityBugs plugin and verified report.
5. Formatted code according to WSO2 code style.
6. Have you verified the PR doesn't commit any keys, passwords, tokens, usernames, or other secrets?
7. Migration scripts written (if applicable).
8. Have you followed secure coding standards in WSO2 Secure Engineering Guidelines?

Testing Checklist

1. Written unit tests.
2. Verified tests in multiple database environments (if applicable).
3. Tested with BI enabled (if applicable).

[aka4rKO]

why did we change this to a string? What if in the future we have a requirement to pass objects around the executor framework? Like for an example we extract the certificate in one executor and set it as a property and do some validations from another executor

[Ashi1993]

Yes I agree, Will keep it as <String, Object> map