Title: Support Body Transformations

Description:.
Envoy supports manipulating/transforming headers, would be great to also support transforming the request and response body to be able to

• sanitize fields
• support API conversions

Relevant Links:

Adding a list of other proxy implementations that support this

• Amazon API Gateway https://docs.aws.amazon.com/apigateway/latest/developerguide/rest-api-data-transformations.html
• Tyk https://tyk.io/docs/transform-traffic/request-body/
• KrakenD https://www.krakend.io/docs/enterprise/backends/body-generator/
• Gloo Edge https://docs.solo.io/gloo-edge/latest/guides/traffic_management/request_processing/transformations/
• Kong https://docs.konghq.com/hub/kong-inc/request-transformer/
• Apache Apisix https://apisix.apache.org/docs/apisix/plugins/body-transformer/
• Apigee https://cloud.google.com/apigee/docs/api-platform/develop/shaping-and-converting-messages