add support for --summarize-prs (WIP)

[migueldiascosta]

This would require pagination (#2776) to do properly, but as it is it will summarize the oldest 100 open PRs by author or label, e.g.

$ eb --summarize-prs
== Summarizing PRs with parameters: direction=asc, per_page=100, sort=created, state=open
boegel: 18
jordiblasco: 7
lexming: 6
henkela: 6
verdurin: 5
SimonPinches: 5
mboisson: 4
fizwit: 4
JackPerdue: 3
ekieffer: 3
akesandgren: 3
smoors: 2
Flamefire: 2
rtripath89: 2
orbsmiv: 2
HPC-UniOldenburg: 2
bartoldeman: 2
zao: 1
cmeesters: 1
tardigradus: 1
alirezaghavaminia: 1
edmondac: 1
vanzod: 1
gppezzi: 1
rokzitko: 1
DininduSenanayake: 1
schulkov: 1
seb45tian: 1
jedokaplan: 1
gmatteo: 1
levlafayette: 1
DinaMah: 1
Darkless012: 1
tstrempel: 1
casio888: 1
guacke: 1
rsdmse: 1
timeu: 1
Micket: 1
ghferrari: 1
asoete: 1

$ eb --summarize-prs=labels
== Summarizing PRs with parameters: direction=asc, per_page=100, sort=created, state=open
update: 34
new: 33
enhancement: 6
bug fix: 3
change: 3
code cleanup: 1
python3: 1

My motivation was that this may help prioritize which open PRs to work on, e.g. the ones of specific authors when we know they are available to work on request for changes, for instance, or specific types (labels) of PRs.

Also, I guess this is generic enough that there are already ways of getting this information from any github repo and I just don't know about them, but this may allow us to look for other things specific to the easyconfigs repo?

In summary, FWIW.