PEM files
---------

You can encode a `HeldCertificate` in PEM format:

```java
HeldCertificate heldCertificate = ...
System.out.println(heldCertificate.certificatePem())
```

```
-----BEGIN CERTIFICATE-----
MIIBSjCB8aADAgECAgEBMAoGCCqGSM49BAMCMC8xLTArBgNVBAMTJDJiYWY3NzVl
LWE4MzUtNDM5ZS1hYWE2LTgzNmNiNDlmMGM3MTAeFw0xODA3MTMxMjA0MzJaFw0x
ODA3MTQxMjA0MzJaMC8xLTArBgNVBAMTJDJiYWY3NzVlLWE4MzUtNDM5ZS1hYWE2