Add an `--allow-hostnames` flag

[dgzlopes]

Feature Description

As of today, k6 supports blocking hostnames based on a list of glob-matching strings. It would also be useful to support the opposite behavior: allowing requests only to a specified set of hostnames.

Suggested Solution (optional)

Add an --allow-hostnames flag:

k6 run --allow-hostnames="api.example.com,*.example.com" script.js

Behavior:

• Only listed hostnames are allowed
• All others are blocked
• Mutually exclusive with --block-hostnames

Already existing or connected issues / PRs (optional)

No response