keys represent the header name as it will be sent over the wire, and * each value is a vec of strings associated with the header. * * While header names are not case-sensitive, getHeaders() will preserve the * exact case in which headers were originally specified. * * Implementations *MUST* return header names of the form `Foo-Bar`, but keys * should be considered case-insensitive. * * Implementations *MAY* choose to normalize some headers in different ways, * for example, `ETag` instead of `Etag`. */ public function getHeaders(): dict<string, vec<string>>;