Fix/auth manager http2 ping keepalive

[etgraylog]

Notes for Reviewers

This PR supercedes PR Graylog2/collector-sidecar#544 to resolve issue #13 .

Summary

• Populates http.Transport.HTTP2 on auth.Manager's default HTTP client with SendPingTimeout=30s and PingTimeout=10s
• Half-dead HTTP/2 connections (upstream silently dropped, no RST) are evicted within ~40s instead of waiting for the kernel's ~11-minute TCP keepalive ladder
• Healthy traffic is unaffected — any inbound frame resets the SendPingTimeout timer
• A caller-supplied HTTPClient in ManagerConfig is left untouched

Test plan

• go test ./superv/auth/... passes

• Confirm GODEBUG=http2debug=2 shows Transport sending health check at the configured interval against a live Graylog server

• The commit history must be preserved - please use the rebase-merge or standard merge option instead of squash-merge

• Sync up with the author before merging