Wire k6provider structured logging into k6

[mstoykov]

What?

Adds a slog.Handler bridge (logrusSlogHandler) that forwards k6provider's
structured log events through k6's existing logrus logger. Provider operations
— artifact resolution, cache hits, downloads, retries, and cache pruning —
now appear in k6's log output at the correct level with structured fields.

Removes the now-redundant manual "A new k6 binary has been provisioned"
Info log and its formatDependencies helper from the provisioner.

Screenshots and log output examples: grafana/k6provider#116

Why?

k6provider gained structured slog logging support (grafana/k6provider#116).
Without this change those log events are silently discarded. Wiring them
through k6's logger makes provisioning behaviour observable without adding
noise for users who don't need it (Debug-level events stay hidden by default).

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Related PR(s)/Issue(s)

• Add structured logging support k6provider#116

Note: k6provider is currently pinned to a pre-release pseudo-version
while grafana/k6provider#116 is in review. The dependency will be updated
to the tagged release before this PR is merged.

• Auto Extension Resolution logging improvement #5525

[codebien]

LGTM, @mstoykov can you rebase plaese?