Support providing direct peers with a file
**Is your feature request related to a problem? Please describe.**
Currently the only way to provide a list direct peers to connect to is using a CLI flag:
```
     --direct-peer             The list of privileged, secure and known peers to connect and maintain the
                               connection to. This requires a not random netkey-file. In the multiaddress
                               format like: /ip4/<address>/tcp/<port>/p2p/<peerId-public-key>, or enr format
                               (enr:-xx). Peering agreements are established out of band and must be
                               reciprocal.
```
Using CLI flag is cumbersome for large lists and easily leads to flag syntax errors.

**Describe the solution you'd like**
I would like a support for `--direct-peers-file` the same way there is `--bootstrap-file`.

**Describe alternatives you've considered**
An API call for adding peers could work, but would require re-adding after every restart.

**Additional context**
I like turtles.