adding api manager keycloak keymanager code

[gayanlggd]

Purpose

Adding initial code base for api manager keycloak keymanager component.

Goals

Provision Keycloak as a third party key manager for WSO2 APIM 2.6

Approach

Keycloak Client is written extending the AbstractKeyManager class.

User stories

Adopting a third party key manager, Keycloak for API Manager 2.6.0

Release note

Third party keymanager implementation for API Manager and Keycloak.

Documentation

Will be added in the future.

Training

N/A

Certification

N/A. Does not lie on the major flaws in APIM since it is custom implementation.

Marketing

N/A

Automation tests

N/A

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Samples

Provide high-level details about the samples related to this feature

Related PRs

N/A

Migrations (if applicable)

N/A

Test environment

JDK 8

Learning

Keycloak official documentation https://www.keycloak.org/docs-api/5.0/rest-api/index.html#_clients_resource

[CLAassistant]

All committers have signed the CLA.