feat: configure default cas backend size

[migmartri]

As mentioned here #618, until now the default cas-backend size was fixed to 100MB, this patch allows you to configure the default value for new backends.

Includes the necessary nobs in the Helm Chart too

 data:
   config.yaml: |
@@ -568,6 +568,7 @@
         addr: foo-chainloop-cas-api:80
       insecure: true
       download_url: /download
+      default_entry_max_size: "200MB"
     plugins_dir: /plugins
     referrer_shared_index:

refs #618