Add IPv6 support for gRPC gun adapter

[KalleJoP]

Summary

Improves IPv6 support for gRPC client connections by (1) falling back to AAAA DNS lookups when no A records exist, and (2) passing IP addresses to Gun as tuples instead of strings.

Why

IPv6 connections were failing with the Gun adapter for two reasons:

1. Missing AAAA lookup — For dns:// targets, only A (IPv4) records were queried. Hosts that expose only IPv6 or that we want to use over IPv6 had no resolved addresses, so the client never attempted an IPv6 connection.
2. Host format for Gun — When an IPv6 address was used (e.g. from a manual config), it was passed to :gun.open/3 as a string. Gun expects IP addresses as inet:ip_address() tuples (e.g. an 8-element tuple for IPv6). Passing a string could lead to failed or unreliable IPv6 connections.

This PR fixes both: DNS now falls back to AAAA when A returns nothing, and we parse IP strings to tuples before calling Gun so IPv6 addresses are in the correct form.

Changes

1. DNS resolver: AAAA fallback (grpc_client/lib/grpc/client/resolver/dns.ex)

• If the A-record lookup returns no addresses, the resolver now falls back to an AAAA lookup so IPv6-only or dual-stack targets can be resolved.
• Logic: try A first; if the result is an empty list, call lookup_addresses(host, :aaaa).

2. Gun adapter: parse host to tuple before open (grpc_client/lib/grpc/client/adapters/gun.ex)

• Before calling :gun.open/3, the host is passed through a new parse_address/1 helper.
• If the host is a valid IP string (IPv4 or IPv6), :inet.parse_address/1 is used and the resulting tuple is passed to :gun.open.
• If parsing fails (e.g. hostname), the host is passed as a charlist as before.
• This ensures IPv6 addresses (e.g. from AAAA) are given to Gun in the form it expects

[sleipnir]

Could you add a test to verify this fallback?

[KalleJoP]

Thanks for the reviews! 👍