SortedList() and FormatForKVStore() can be very hot code in environments
where there's constant policy churn, especially CIDR policies where
there can be a large number of CIDR labels.

This commit adds benchmarks for later commits to use as a baseline.

Signed-off-by: Chris Tarazi <chris@isovalent.com>

FormatForKVStore() previously returned a string for no reason as every
caller converted the return value to a byte slice. This allows us to
eliminate string concatenation entirely and use the bytes.Buffer
directly.

Building on the above, given that SortedList() returns a byte slice and
calls FormatForKVStore() for its output, we can optimize it with the
same technique to eliminate string concatenation.

Here are the benchmark comparisons:

```
$ go test -v -run '^$' -bench 'BenchmarkLabels_SortedList|BenchmarkLabel_FormatForKVStore' -benchmem ./pkg/labels > old.txt
$ go test -v -run '^$' -bench 'BenchmarkLabels_SortedList|BenchmarkLabel_FormatForKVStore' -benchmem ./pkg/labels > new.txt
$ benchcmp old.txt new.txt
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark                             old ns/op     new ns/op     delta
BenchmarkLabels_SortedList-8          2612          1120          -57.12%
BenchmarkLabel_FormatForKVStore-8     262           54.5          -79.18%

benchmark                             old allocs     new allocs     delta
BenchmarkLabels_SortedList-8          35             13             -62.86%
BenchmarkLabel_FormatForKVStore-8     4              1              -75.00%

benchmark                             old bytes     new bytes     delta
BenchmarkLabels_SortedList-8          1112          664           -40.29%
BenchmarkLabel_FormatForKVStore-8     96            48            -50.00%
```

Signed-off-by: Chris Tarazi <chris@isovalent.com>