WFE2: support POST-as-GET for directory/newNonce endpoints

WFE2: support POST-as-GET for directory/newNonce endpoints: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Presently Boulder only supports GET requests for the directory endpoint and GET/HEAD requests for the newNonce endpoint.

The task

Presently Boulder only supports GET requests for the directory endpoint and GET/HEAD requests for the newNonce endpoint.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.