iOS: paging: create metadata resolver strategy for header

The 'set metadataLocation' on the AGPipe defines WHERE the client receives the information about the next/previous links.

This task focuses on reading them from (custom) http response headers

See also nextIdentifier/previousIdentifier on the spec (where the names are provided)

HEADERS.... there may/must be a "NEXT HEADER" and a separate HEADER for the "previous" link.... the names are defined by the nextIdentifier/previousIdentifier config...