Add SSL termination to the explorer

Description

Manage SSL termination directly from the explorer, to remove the reverse-proxy dependency (caddy)

Changes

• explorer/lib/explorer_web/router.ex: make CSP less strict for the website to be able to:
  • 'unsafe-inline': execute JS scripts inline
  • img-src https://w3.org blob: data:;: fetch some image/data from w3
• explorer/config/runtime.exs: handle SSL certificates from elixir endpoint
  • PORT_SSL env var sets the port for HTTPS endpoint to listen on, not needed as it's defaulted to 443
    • for the 443 binding to work, it's needed to either:
      • run the release with sudo (not recommended for security reasons)
      • add capabilities to the BEAM binary to bind to low ports, for example in staging: sudo setcap CAP_NET_BIND_SERVICE=+eip /home/app/repos/explorer/aligned_layer/explorer/_build/prod/rel/explorer/erts-14.2.1/bin/beam.smp
  • KEYFILE_PATH and CERTFILE_PATH are the local paths to the SSL private key and the certificate, if not set, HTTP endpoint will be brought up, but not HTTPS one
  • NOTE: as this config is all inside the if config_env() == :prod, local development setup changes are not needed