examples: add writing and reading ca files

[iamjpotts]

Adds an example based on 0.14 that generates a certificate chain, and stores each step as PEM files, and reads from the PEM files for the next step, rather than doing all of the steps in memory.

Related:

• Signing leaf certificates with parsed intermediate #268
• Add Certificate::from_der + Certificate::from_pem #274
• How to create a certificate signed by an existed CA certificate? #354
• CertifiedIssuer::from_ca_cert_{pem,der}? #375
• Add example of generating a new CA and a new server cert signed by it #99

@oscartbeaumont said he "wanted to know if there is a way to load an existing CA certificate to sign some more certificates without triggering signing."

@acul009 mentioned the api obscurity as discouraging an upgrade from 0.12 to 0.13 in #274 (comment).

This new example does not resign any certificates, instead loading the already signed certificate as an Issuer.

[djc]

Thanks for your efforts. I'm generally pretty wary of adding too many examples since they can add maintenance costs. In this case, your example is pretty long already (200+ lines) and IMO working directly on the file system makes it less generalizable (and more likely to leave clutter around if you actually try to run it).

While I agree it could be useful to have a specific example that shows how to load an existing CA certificate without resigning, I'd prefer a more concise and focused piece of code for that.

[iamjpotts]

@djc see #379 as an alternative example to this example.

[iamjpotts]

Force push only renames new_unsigned_server to new_signed_server to match its behavior.