Add host file input

[klauspost]

Allow file as host input for benchmark targets or clients.

Example warp-hosts.txt:

127.0.0.1:{9001...9004}

127.1.2.3
giga.abc.com

Debug printing loaded hosts:

λ warp stat --host=file:warp-hosts.txt
HOSTS: [127.0.0.1:9001 127.0.0.1:9002 127.0.0.1:9003 127.0.0.1:9004 127.1.2.3 giga.abc.com]

λ warp stat --warp-client=file:warp-hosts.txt
hosts [127.0.0.1:9001 127.0.0.1:9002 127.0.0.1:9003 127.0.0.1:9004 127.1.2.3 giga.abc.com]

Slightly more generic version of https://github.com/minio/warp/pull/297 (and avoids another flag)