Feature: add config file option to read CA cert from file

[sseide]

Currently setting extra CA certs is possible only in the ollowing ways:

• adding certificate directly as one-liner inside the config file or as env var
• for single connections completly configured by environment variables a file name can be given

Improve RC to add more possiblities to read a custom CA cert from file too:

• add json config file parameter to read cert from file before giving the tls object to "ioredis" constructor. Somethig like

  "tls": {
    "caCertFile": "/data/my-ca-cert.pem"
    ...
  }

• use cert file read from env var for all connections(?)
• add reference to NodeJS env var: NODE_EXTRA_CA_CERTS to the documentation as alternative: https://nodejs.org/api/cli.html#cli_node_extra_ca_certs_file

The last to point - overlap? Only one needed? Check neccessary.

Issue created from #554