Provide structure for using multiple CAs

Provide structure for multiple CA support. This mitigates the need for per-SKU, and supports section 3.2.2 whereby the common CA operation path

```bash
/.well-known/est/cacerts
```

further to this, individual CAs can then be retrieved by means of a label reference.

```bash
/.well-known/est/arbitraryLabel1/cacerts
```

Which is defined on page 17.
