Add Jason as dependency and other improvements

Changed

• Always use Jason to encode JSON. This removes the need to install either Jason or Poison alongside AppSignal, simplifying our installation instructions.
• Filter more disk mountpoints for disk usage and disk IO stats. This helps reduce noise in the host metrics by focussing on more important mountpoints. Any mountpoint containing /etc/hostname, /etc/hosts, /etc/resolv.conf, /snap/ or /proc/ is ignored.

Fixed

• Support disk usage reporting (using df) on Alpine Linux. This host metric would report an error on Alpine Linux.
• When a disk mountpoint has no inodes usage percentage, skip the mountpoint, and report the inodes information successfully for the inodes that do have an inodes usage percentage.

See the changelog for AppSignal for Elixir package 2.8.0 for more information.