Support the use of client certificates

[dylan-smith]

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.

[spindev]

@dylan-smith I will have a look at this. Feel free to assign it to me :)

[robfreud]

Hi team - This is a high priority for a large initiative (ARR) customer. See above with @djredman99 's link