feat(update): add --type flag to bd update command (GH#522)

Allow changing an issue type (bug, feature, task, epic, chore) via the
bd update command. The storage layer already supported issue_type in
allowedUpdateFields, this just exposes it through the CLI.

Changes:
- Add --type/-t flag to updateCmd in show.go
- Add IssueType field to UpdateArgs in protocol.go
- Handle issue_type in updatesFromArgs in server_issues_epics.go
- Add validation using ParseIssueType before update

Example usage:
  bd update ab-xyz --type epic

Fixes: #522

internal/rpc/server_issues_epics.go

@@ -73,6 +73,9 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.EstimatedMinutes != nil {
 		u["estimated_minutes"] = *a.EstimatedMinutes
 	}
+	if a.IssueType != nil {
+		u["issue_type"] = *a.IssueType
+	}
 	return u
 }