forge: Genericize github handling into a forge abstraction

[jerry-skydio]

Clean up and isolate the github specific code behind a forge
abstraction. This makes it possible to add tests with dummy forge
backends and also makes it easier to add more forges in the future.

Update comments, prints, and docs where necessary.

--github-oauth and --github-url remain as backward compatibility
for forge-oauth and forget-url. github-username has been removed
completely as it hasn't been used in a long time.

[jerry-skydio]

Reviews in this chain:
└#249 forge: Genericize github handling into a forge abstraction
　└#250 test: Expand upload tests with fake_forge
　　└#253 github: Add graphql utility class to contain query fields
　　　└#254 github: Add request retry splitting

[jerry-skydio]

#	head	base	diff	date	summary
0	2bc698e6	d937b197	diff	May 19 17:58 PM	21 files changed, 1106 insertions(+), 1116 deletions(-)
1	3b0639b4	d937b197	diff	May 19 18:13 PM	5 files changed, 69 insertions(+), 67 deletions(-)
2	bd7abad6	d937b197	diff	May 19 18:15 PM	0 files changed
3	98287168	d937b197	diff	May 20 14:15 PM	1 file changed, 1 insertion(+), 4 deletions(-)

[brian-kubisiak-skydio]

nit: Technically this only works for "github-like" forges (gitlab, forgejo, etc) and won't work for e.g. sourcehut, radicle, or gerrit. It's also unlikely revup would ever support those though.