add the rich media ttl based on image

config/config.exs

docs/config/hotto_richmedia_cache_ttl_based_on_image.md

lib/pleroma/web/rich_media/parser.ex

lib/pleroma/web/rich_media/parsers/ttl/aws_signed_url.ex

lib/pleroma/web/rich_media/parsers/ttl/mfc_signed_url.ex