feat(datasource/hex): Add V2 registry protocol support

[zharinov]

Changes

Add Hex V2 registry protocol support as a secondary code path alongside the existing JSON API. This enables Renovate to fetch package data from third-party Hex registries (like Oban Pro, corporate registries) that only implement the Hex V2 registry protocol and have no JSON API.

Dispatch logic: hex.pm (default registry) → JSON API (unchanged), any other registry URL → V2 protocol.

The V2 decode pipeline: getBuffer() → gunzipSync → Signed.decode → Package.decode → map to ReleaseResult. Packages fetched via V2 have versions and deprecation status but no timestamps, sourceUrl, or homepage (this metadata doesn't exist in V2 or on custom registries).

This PR adds the decoding capability only. A follow-up PR will add end-to-end custom registry support (parsing repo: in mix.exs, mix hex.repo add in artifacts, wiring registry URLs through config).

Context

• Support hex v2 registry #29622

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