add convert command to sshproxyctl

[cyrilst]

• The hack made for unmarshaling durations was not needed since about 10 years. I removed it and cleaned a bit the code.
• No need to add a test in docker, as configv1_test.go tests the functionality from end to end (input file to output text)

[github-actions]

File	Coverage
All files	6%	✅
cmd/sshproxy-dumpd/sshproxy-dumpd.go	0%	✅
cmd/sshproxy-replay/sshproxy-replay.go	0%	✅
cmd/sshproxy/commands.go	0%	✅
cmd/sshproxy/recorder.go	0%	✅
cmd/sshproxy/sshproxy.go	0%	✅
cmd/sshproxyctl/sshproxyctl.go	0%	✅
pkg/nodesets/nodesets.go	0%	✅
pkg/record/record.go	0%	✅
pkg/utils/config.go	0%	✅
pkg/utils/configv1.go	100%	✅
pkg/utils/etcd.go	0%	✅
pkg/utils/logging.go	0%	✅
pkg/utils/route.go	0%	✅
pkg/utils/utils.go	92%	✅

Minimum allowed coverage is 0%

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