Custom headers are not available to the SNI `override_auto_sni_header` when added using `request_headers_to_add`

## Title

Custom headers are not available to the SNI `override_auto_sni_header` when added using `request_headers_to_add`

## Description

We now allow a customer header to populate the SNI for the upstream requests [[Link]](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/protocol.proto#:~:text=with%20auto_sni%20field.-,override_auto_sni_header,-(string)%20An%20optional). But, if the custom header is not coming from downstream and gets added anywhere using `request_headers_to_add` then it's value is not available here as [this](https://github.com/envoyproxy/envoy/blob/5b1015cffbf6e02a7ccec9c12a6b7f246f8f3d30/source/common/router/router.cc#L517) piece of code to populate the SNI gets invoked before we add/modify the request headers [probably here](https://github.com/envoyproxy/envoy/blob/5b1015cffbf6e02a7ccec9c12a6b7f246f8f3d30/source/common/router/router.cc#L648).