We've seen in cert-manager#7506 that the PEM size maxima we have can be too
restrictive for some users who have large numbers of DNS identities in
their leaf certificates, preventing them from using cert-manager for
those certificates (or stopping them from upgrading).

This change adds a large estimate for extra data which might be present
in a leaf certificate, based on an experimentally observed increase in
the size of a certificate request object by adding new DNS names.

We assume 250 large DNS names, totalling about 30kB of size.

We shouldn't expect to see a performance change in our existing
benchmark with these changes, because the maxBundleSize is used for
those tests and that size remains larger than the new
maxCertificateChainSize. That seems to be the case from testing:

```console
$ go test -bench=. ./internal/pem/...
goos: darwin
goarch: arm64
pkg: github.com/cert-manager/cert-manager/internal/pem
cpu: Apple M3 Max
BenchmarkPathologicalInput-16                 52          22606764 ns/op
PASS
ok      github.com/cert-manager/cert-manager/internal/pem       2.485s

$ go test -bench=. ./internal/pem/...
goos: darwin
goarch: arm64
pkg: github.com/cert-manager/cert-manager/internal/pem
cpu: Apple M3 Max
BenchmarkPathologicalInput-16                 52          22455893 ns/op
PASS
ok      github.com/cert-manager/cert-manager/internal/pem       2.408s
```

Signed-off-by: Ashley Davis <ashley.davis@cyberark.com>