2.7.2rc15: Feature improvements and documentation updates

What's Changed

• [2.7] Added fix for RCE bug caused by not validating type_name by @nvidianz in #4295
• Add missing types for FOBS by @nvidianz in #4298
• [2.7] Fix FilePipe TOCTOU by @YuanTingHsieh in #4296
• [2.7] Fix pt_init_client data_path to avoid concurrent CIFAR10 download corruption by @YuanTingHsieh in #4297
• [2.7] Datasets license; swarm readme update [skip ci] by @holgerroth in #4303
• [2.7] Docs update to include CVM base image building instructions by @nvidianz in #4306
• [2.7]: Sign the root folders of startup kits by @IsaacYangSLA in #4301
• [2.7]: Fix an inconsistency between document and codes by @IsaacYangSLA in #4308
• [2.7] Receiver-side PASS_THROUGH for ext-process CJ forward path by @YuanTingHsieh in #4309
• [2.7] increase FilePipe default heartbeat timeout value to 600s by @YuanTingHsieh in #4307

Full Changelog: 2.7.2rc14...2.7.2rc15