Add cache to /mapping/entity endpoint to improve loading experience for PagerDutyPage

[t1agob]

Is your feature request related to a problem? Please describe.
The mapping/entity endpoint exposed by the backend component builds a list of all mappings between services and Backstage entities. Depending on the number of PagerDuty Services this might take a long time.

In #75, @alpinweis reported than in their case with over 200 services it takes consistently 10-15 seconds to load the page.

Describe the solution you'd like
I would like the performance of this endpoint to be improved drastically otherwise the user experience is not great.

There are users with a lot more services across several PagerDuty accounts and they cannot wait indefinitely for the page to load. This information needs to be cache because most of it is fairly static.

Describe alternatives you've considered
At this point in time there is no other option than to wait for the page to load.