Access header values by name from header modification options

[taion809]

Issue Template

Title: Access header values by name from header modification options

Description:
Today it is not possible to rename or set a header value based on the value of an existing header. I propose adding %REQ("header_key")% to the header parser. This will allow us to automatically upgrade legacy headers in older applications.

envoy/source/common/router/header_formatter.cc

Line 210 in 818fc0c

throw EnvoyException(fmt::format("field '{}' not supported as custom header", field_name));

[stale]

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[stale]

This issue has been automatically closed because it has not had activity in the last 37 days. If this issue is still valid, please ping a maintainer and ask them to label it as "help wanted". Thank you for your contributions.

[mohammadMahdavi]

I also want to rename the existing heather. Is there any solution?