Function is_accept_multipart_mixed

pub fn is_accept_multipart_mixed(accept: &str) -> bool

Check accept is multipart-mixed

Example header

Accept: multipart/mixed; boundary="graphql"; subscriptionSpec="1.0"

the value for boundary should always be graphql, and the value for subscriptionSpec should always be 1.0.

Reference: https://www.apollographql.com/docs/router/executing-operations/subscription-multipart-protocol/