Accept multiple {firehose,subtreams}-grpc-listen-addr

[maoueh]

The various Substreams & Firehose "public" endpoint flags that bind gRPC/HTTP server(s) should accept a list of addresses to listen on. That way, it would be possible to listen for both secure and insecure on 2 different ports.

The implementation would be to use StringArray. It should stay backward compatible so that current config that has just one element still works. Here the test cases:

• --firehose-grpc-listen-addr=localhost:9000
• --firehose-grpc-listen-addr=localhost:9000*
• --firehose-grpc-listen-addr=localhost:9000*,localhost:8000
• --firehose-grpc-listen-addr=localhost:9000* --firehose-grpc-listen-addr=localhost:8000

In the 2 last cases, both 8000 and 9000 ports should be open, both serving their respective service. The 9000 would be TLS insecure using our snake oil certificate, 8000 would be in plain-text.

[maoueh]

@sduchesneau Is this now completed fully?