Use strings.Builder instead of fmt.Sprintf() which is known to be not
performant.

This optimization was identified by inspecting a pprof (memory profile)
of a cluster with a wildcard L7 DNS rule policy (matchPattern: '*') and
a workload selected by the policy making a large amount of unique DNS
requests.

```
$ go test -v -run '^$' -bench 'Benchmark_maskedIPNetToLabelString' -benchtime 50000x -benchmem ./pkg/labels/cidr > old.txt
$ go test -v -run '^$' -bench 'Benchmark_maskedIPNetToLabelString' -benchtime 50000x -benchmem ./pkg/labels/cidr > 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
Benchmark_maskedIPNetToLabelString-8     8122          4450          -45.21%

benchmark                                old allocs     new allocs     delta
Benchmark_maskedIPNetToLabelString-8     39             32             -17.95%

benchmark                                old bytes     new bytes     delta
Benchmark_maskedIPNetToLabelString-8     488           368           -24.59%
```

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

Avoid using fmt.Sprintf() when a simple string concatentation does the
same job, and preallocate slices where possible.

These optimizations were identified by inspecting a pprof (memory
profile) of a cluster with a wildcard L7 DNS rule policy (matchPattern:
'*') and a workload selected by the policy making a large amount of
unique DNS requests.

Without the previous commit that optimizes the maskedIPToLabelString():

```
$ go test -v -run '^$' -bench 'Benchmark_GetCIDRLabels' -benchtime 5000x -benchmem ./pkg/labels/cidr > old.txt
$ go test -v -run '^$' -bench 'Benchmark_GetCIDRLabels' -benchtime 5000x -benchmem ./pkg/labels/cidr > 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
Benchmark_GetCIDRLabels-8     391590        390163        -0.36%

benchmark                     old allocs     new allocs     delta
Benchmark_GetCIDRLabels-8     1455           1424           -2.13%

benchmark                     old bytes     new bytes     delta
Benchmark_GetCIDRLabels-8     64143         63189         -1.49%
```

With previous commit:

```
benchmark                     old ns/op     new ns/op     delta
Benchmark_GetCIDRLabels-8     390163        295259        -24.32%

benchmark                     old allocs     new allocs     delta
Benchmark_GetCIDRLabels-8     1424           1131           -20.58%

benchmark                     old bytes     new bytes     delta
Benchmark_GetCIDRLabels-8     63189         58707         -7.09%
```

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