Support multiple certs in configuration

Currently amppackager has support for only 1 cert file. For a multi domain web property,

e.g.
example.org
subdomain.example.org
example.uk

Setting up amppackager would mean creating a config for every subdomain or TLD. The amppkg.toml should allow for setting the mapping of cert file to supported domains. An example of this is

CertFile = '.../global-cert.pem'
KeyFile = '.../global-key.pem'
...

[URLSet.Sign]
CertFile = '.../cert-0.pem' # Override the global Certfile
KeyFile = '.../privkey-0.pem' # Override the global KeyFile
Domain = 'example.org'
...
[URLSet.Sign]
CertFile = '.../cert-1.pem' # Override the global Certfile
KeyFile = '.../privkey-2.pem' # Override the global KeyFile
Domain = 'example.uk'
...
[URLSet.Sign]
# Use the global Certfile and KeyFile
Domain = 'example.org'
...

Start by reading the amppkg.toml configuration handling and the URLSet.Sign settings described in the issue. Trace how global CertFile and KeyFile values are selected, then verify that per-domain overrides and global fallback work for the example.org, subdomain.example.org, and example.uk cases.

Written by the indexing model from the issue text.