Populate Access-Control-Expose-Headers for cross component correlation

[iusafaro]

Populate Access-Control-Expose-Headers with Request-Context value to allow read value of Request-Context header from JavaScript SDK and use it for target in AJAX dependencies allowing cross-component correlation between AJAX dependencies and server-side requests.

https://www.w3.org/TR/cors/#http-access-control-expose-headers

[TimothyMothra]

One thing to note, this change is appending a new Value to the Header Key:

I don't know if this is intended. The HTTP standard might prefer comma delimited values:

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2