Support the use of client certificates

## Description

When communicating with customers on-prem servers (e.g. BBS or GHES) it's possible they have configured them to require client certificates (we had at least one ticket from a BBS customer that was blocked from using bbs-exporter because it doesn't support client certificates).

We could add some extra args to support client certs in `bbs2gh` and `gei`. `HttpClient` already has support, we just need to do a bit extra configuration.