feat: Add typed Copilot metrics download helpers

Add DownloadDailyMetrics, DownloadPeriodicMetrics,
DownloadUserDailyMetrics, and DownloadUserPeriodicMetrics to
decode the payloads served at the download links returned by
the new Get*MetricsReport endpoints. User metrics payloads are
newline-delimited JSON; aggregate payloads are JSON objects
(1-day) or a { day_totals: [...] } wrapper (28-day).

Deprecate DownloadCopilotMetrics because the []*CopilotMetrics
shape it produces does not match the new payloads. It is kept
for GitHub Enterprise Server installations that may still serve
the legacy shape.

Fixes #4136

Author: kyungseopk1m