Add Sign1Message benchmarks

[qmuntal]

This PR adds benchmarks for Sign1Message methods. These benchmarks will be useful later one to do memory and cpu profiling.

These are the results on my Windows laptop (use only for reference):

goos: windows
goarch: amd64
pkg: github.com/veraison/go-cose
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
BenchmarkSign1Message_MarshalCBOR-12              666874              1791 ns/op             692 B/op         17 allocs/op
BenchmarkSign1Message_UnmarshalCBOR-12            368671              3303 ns/op            1592 B/op         28 allocs/op
BenchmarkSign1Message_Sign-12                     631834              1739 ns/op             668 B/op         16 allocs/op
BenchmarkSign1Message_Verify-12                   585108              1910 ns/op             668 B/op         16 allocs/op

For #52