feat: Allow add para to running network without register (#170)

- Add `custom_parchain_fs_prefix` parameter to `add_para` fn (on running
network), this allow to manage the fs layout and allow to add paras with
the same id.
- Minor clippy auto-fixes

cc: @metricaez

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Co-authored-by: Loris Moulin <45130584+l0r1s@users.noreply.github.com>