Add support for listing issues for a specific revision or project

## Why

`fossapi list revisions ` exists, and `fossapi get revision
` returns revision metadata — but there is no way to list the
issues attached to a revision. Per-project issues are the second half of a
release-group reconciliation (see sibling issue on release-group issue
listing), and they're also useful standalone — e.g. "what are the current
license findings on this revision?"

## Current workaround

```bash
REV='custom+/$'
ENC=$(python3 -c "import urllib.parse,sys;print(urllib.parse.quote(sys.argv[1],safe=''))" "$REV")
curl -sH "Authorization: Bearer $FOSSA_API_KEY" \
"https://app.fossa.com/api/revisions/$ENC/issues"
```

Response shape: `{revisionScan: {...}, issues: [{id, type, revisionId,
humanReadableType, vulnId, ruleId, licenseForDefaultRule, scan_type,
ignoreData, parents, ...}]}`. Issue rows have `scan_type ==
"top_level_revision"`, as opposed to `"project_group_release"` on the RG
endpoint — worth surfacing in the CLI's JSON output.

## Proposed CLI surface

```
fossapi list revision-issues [--json] \
[--category licensing|vulnerability|quality]
# or an equivalent filter on `list issues`:
fossapi list issues --revision [--category <...>]
```