add the rich media ttl based on image exp time

docs/config/howto_set_richmedia_cache_ttl_based_on_image.md

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+# How to set rich media cache ttl based on image ttl
+## Explanation
+
+Richmedia are cached without the ttl but the rich media may have image which can expire, like aws signed url.
+In such cases the old image url (expired) is returned from the media cache.
+
+So to avoid such situation we can define a moddule that will set ttl based no image.
+
+The module must have a `run` function and it should be registered in the config.
+
+### Example
+
+```exs
+defmodule MyModule do
+  def run(data, url) do
+    image_url = Map.get(data, :image)
+    # do some parsing in the url and get the ttl of the image
+    # ttl is unix time
+    ttl = parse_ttl_from_url(image_url)  
+    Cachex.expire_at(:rich_media_cache, url, ttl * 1000)
+  end
+end
+```
+
+And update the config
+
+```exs
+config :pleroma, :rich_media,
+  ttl_setters: [Pleroma.Web.RichMedia.Parser.TTL.AwsSignedUrl, MyModule]
+```
+
+> For reference there is a parser for AWS signed URL `Pleroma.Web.RichMedia.Parser.TTL.AwsSignedUrl`, it's enabled by default.