feat: implemented basic cache of 30 sec in can-i-merge badges

[pahnin]

• currently the can-i-merge badges are not cached at al
• if a badge is present it makes sense to have some basic cache (30sec) instead of none

[vwong]

Looks fine, but I'm not sure how users may react to this...

[mefellows]

Thanks for doing this!

If this were on the can-i-merge API resource itself I’d say caching would be a bad idea, but given it’s on the badge resource I think we could even be a little more generous - perhaps 60s?

For example, if this was part of a CI/CD pipeline and the CLI was giving incorrect (in particular, a false positive) then that would be a problem.

I don’t expect people to be relying on this badge to unblock releases.

Would be interesting to test what impact it has in the dev environment if it helps the flicker, perhaps a small cache time is sufficient to solve that problem?