feat: Add discovery support

[lufik]

I think it's good practice to give the machine owners possibility to define also discoveries.

I added /discovery endpoint in this PR which is configured in config file:

discovery:
  files:
  - <fileglob>
  configs:
  - module: http_2xx
    targets:
    - <host/url>

It should be used in prometheus this way for scraping:

- job_name: "exported-blackbox-probes"
  http_sd_configs:
  - url: http://prometheus-blackbox-exporter:9115/discovery