Add support to publish custom attributes to ELK based Analytics

[gayaldassanayake]

Problem

Currently, it is not possible to publish custom attributes to the ELK based Analytics solution as it is impossible to include any attributes to the existing event schema.

Related internal support issue - https://github.com/wso2-enterprise/wso2-mi-internal/issues/995

Proposed Solution

Allow passing additional properties into a property bag using a custom data provider.

A similar feature has been already implemented in APIM.
Issue - wso2/api-manager#443
PR - wso2/carbon-apimgt#11349

Alternatives

No response

Version

4.3.0