syncthing: add extraFlags option that adjust service
syncthing: add extraFlags option that adjust service

This is useful for situations in which you might want to reset certain
things using `--reset-database` or `--reset-deltas` or debug certain
things using any of the debug options like `--debug-perf-stats`.

Signed-off-by: Jakub Sokołowski <1d3764b91b902f6b45836e2498da81fe35caf6d6@status.im>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs