use diskcache library for caching

[AUTOMATIC1111]

Description

• uses diskcache library for cache instead of a json file as it previously did
• each subsection of cache (safetensors-metadata, hashes, extensions-git...) is its own diskcache.Cache object
• uses SD_WEBUI_CACHE_DIR environment variable, defaults to cache directory
• old json cache is automatically converted if it exists
• the file for old cache is not removed for now, but is unused otherwise

Alternative to #12863. @w-e-w @akx @freecoderwaifu

[w-e-w]

LGTM