Add option for beacon auth header

[Tristan-Wilson]

Testing done

Add a dummy value to the auth header in nodeconfig:

{
  "parent-chain": {
          "blob-client": {
                  "beacon-url": "http://localhost:3509",
                  "authorization": "Basic asdf"
          },
...

Run netcat to intercept beacon chain requests, check that the header is there:

$ nc -l -p 3509
GET /eth/v1/beacon/genesis HTTP/1.1
Host: localhost:3509
User-Agent: Go-http-client/1.1
Authorization: Basic asdf
Accept-Encoding: gzip

[PlasmaPower]

LGTM

[Tristan-Wilson]

Fixes #2159