feat(config): added reason field

[shanduur]

Tested via following command:

./artifacts/grype \
    -c ./test-config.yaml \
    -t ./test-template.tmpl \
    -o template \
    registry.access.redhat.com/ubi8/ubi@sha256:45d0e45bb7e60cb8d9349e579a849579841116f1e4507bb2b977ac83c503e110

Where the config is:

ignore:
  - vulnerability: CVE-2023-4641
    reason: This vulnerability is not relevant in our application, because the vulnerability is only a problem under certain circumstances, that are not present in our application.

And template is:

<p>
    The following vulnerabilities are considered irrelevant:
    <ul>
        {{- range .IgnoredMatches}}
        <li>{{.Vulnerability.ID}} ({{ range $air := .AppliedIgnoreRules }}{{ $air.Reason }}{{ end }})</li>
        {{- end}}
    </ul>
</p>

Obtained output:

$ ./artifacts/grype -c ./test-config.yaml -t ./test-template.tmpl -o template registry.access.redhat.com/ubi8/ubi@sha256:45d0e45bb7e60cb8d9349e579a849579841116f1e4507bb2b977ac83c503e110
 ✔ Vulnerability DB                [no update available]  
 ✔ Parsed image                                                                   sha256:9de842ac8fcc2abf935f65425de90acde7a8a4841cfe9a5c06f0280d9c880681
 ✔ Cataloged packages              [206 packages]  
 ✔ Scanned for vulnerabilities     [212 vulnerability matches]  
   ├── by severity: 3 critical, 2 high, 72 medium, 131 low, 0 negligible (4 unknown)
   └── by status:   0 fixed, 212 not-fixed, 0 ignored 
<p>
    The following vulnerabilities are considered irrelevant:
    <ul>
        <li>CVE-2023-4641 (This vulnerability is not relevant in our application, because the vulnerability is only a problem under certain circumstances, that are not present in our application.)</li>
    </ul>
</p>

The template is not exactly as simple as @gutschet described, but it allows querying the reason for ignoring particular match.

Closes #1337

[shanduur]

@willmurphyscode pinging to check if that's what was expected as a result from that issue.

[willmurphyscode]

@shanduur Thanks very much for the PR! Taking a look today.