Define APIML extensions contract

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Is your feature request related to a problem? Please describe. The number of APIML extensions is growing and customers could develop their own proprietary extensions. Zowe/APIML should make it possible to preserve customers efforts extending Zowe/APIML. Currently APIML extensions lifecycle depends on internal APIML API. This doesn’t imply contract and consequently can bring unexpected issues when the API is changed.

Describe the solution you'd like

• Define APIML extensions contract.
• Consider also packaging and deployment aspects
• Document and publish the APIML extensions lifecycle contract.

Describe alternatives you've considered

• Extensibility mechanism may differ as per APIML sub-components

Additional context

• Soon coming changes of Java version and Netflix components (Zuul, etc.) may significantly impact the extensions API.

• Check the state of the discussion about extensions install/deploy in the System squad (consult Zen Hub).

• See: Zowe configuration workflow?

• Check Jakub's work on a document about Extensions conformance.

• Related issue: https://app.zenhub.com/workspaces/community-5c93e02fa70b456d35b8f0ed/issues/zowe/api-layer/1682

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