Introduce CORS support for REST API

[arunans23]

Purpose

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Currently, CORS cannot be configured in the REST API which is useful when invoking the APIs direclty from modern browsers.

Approach

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This PR brings an improvement to read CORs configuration from synapse.properties file and set in the API request and response path.

The cors can be configured using the following properties in synapse.properties.

synapse.rest.CORSConfig.enabled=true
synapse.rest.CORSConfig.Access-Control-Allow-Origin=http://localhost:3000,https://example.com
synapse.rest.CORSConfig.Access-Control-Allow-Headers=Content-Type,Authorization,X-Requested-With

Release note

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[isudana]

Hi Arunan, Thank you for the contribution. Can you please provide test cases as well?