Add support for ingesting intermediate and root CA certificates via "path" flag(s)

[atc0005]

Not sure yet whether one flag (e.g., --certs-path) would be used to ingest both types, or whether there would be separate flags for each type.

For example:

• --intermediate-certs-path
• --root-certs-path

The current thinking is that regardless of what they're named, the flags would allow pointing directly at a specific file for ingest such as:

• /etc/ssl/certs/ca-certificates.crt (e.g., Ubuntu)
• /etc/ssl/certs/ca-bundle.crt (e.g., RHEL)

or for an entire directory:

• /etc/ssl/certs/

Each ingested file would be sorted into a collection based on its type:

• intermediate certificates
• root certificates
• leaf certificates
  • intentionally ignored

The error handling would probably opt to skip over any file ingest attempts that fail (assuming instead that the file isn't a certificate). Not sure if we'd offer a flag to not ignore ingest errors. Presumably we'd log ingest failures via debug level log messages to aid a sysadmin in troubleshooting why an intended file was not ingested.

[atc0005]

I'm on the fence whether it is worth adding this logic now while I'm in the process of trying to get the new Chain Integrity validation check changes stabilized or after as a refinement of the logic.

[atc0005]

From curl --help:

     --cacert <file> CA certificate to verify peer against
     --capath <dir>  CA directory to verify peer against

Maybe --ca-path for a general "ingest" flag name?

[atc0005]

Debating whether to make the new flag act the same as other multiple input flags for this project and accept paths as a comma-separated value:

/path/to/check_cert --server www.example.com --ca-path "/path/to/intermediates/dir,/path/to/root-ca-certs/dir,/etc/ssl/certs/ca-certificates.crt"

or have it be used multiple times:

/path/to/check_cert --server www.example.com --ca-path "/path/to/intermediates/dir" --ca-path "/path/to/root-ca-certs/dir" --ca-path "/etc/ssl/certs/ca-certificates.crt"

or have the flag support a mix of either:

/path/to/check_cert --server www.example.com --ca-path "/path/to/intermediates/dir,/path/to/root-ca-certs/dir" --ca-path "/etc/ssl/certs/ca-certificates.crt"

Flexibility is good, but consistency is probably better?