easyeffects: add option to import presets

[hauskens]

Description

• added options to import presets
• added tests
• added hausken as maintainer

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.
  (tested with nix-shell --pure tests -A run.easyeffects-service and nix-shell --pure tests -A run.easyeffects-example-preset)

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@fufexan