Enable configuration of formatter_path

[jdurand]

This PR adds the ability to configure the formatter_path setting in neotest-rspec, allowing users to specify custom paths for their RSpec formatter. This enhancement provides more flexibility for projects with non-standard RSpec setups, such as running rspec from a Docker container.

docker-compose.yml:

services:
  test:
    volumes:
      - ~/.local/share/nvim/lazy/neotest-rspec:/neotest-rspec:ro

Refer to my neotest configuration for an example.