Support IDN targets via punycode conversion

[locker95]

Description

Unicode hostnames such as http://мтр24.рф fail DNS with no such host, while the punycode form works. Blackbox resolves hosts itself, so IDN never reaches Go's net/http IDNA handling.

Convert hostnames to ASCII with golang.org/x/net/idna (already a dependency via x/net) before:

• DNS resolution (chooseProtocol — covers HTTP/TCP/ICMP/DNS/gRPC targets)
• HTTP Host / default TLS SNI
• DNS query_name and DoT SNI
• TCP / gRPC default SNI

ASCII names and IP addresses are left unchanged. Logs keep the original target label; only the resolved name is rewritten.

How to verify

go test ./prober/ -run TestIdnaToASCII -count=1
go test ./prober/ -short -count=1

# optional: probe an IDN target
# curl 'http://localhost:9115/probe?module=http_2xx&target=http://мтр24.рф&debug=true'

[ENHANCEMENT] Convert IDN hostnames to punycode so HTTP/DNS/TCP/ICMP/gRPC probes resolve Unicode targets #626

Fixes #626