extsvc: unify OAuth and username/password authentication methods in outbound requests

Created by: LawnGnome

Right now, all the external service client implementations have their own versions of adding authentication headers to outbound requests. In practice, there's a lot of commonality between the methods: an OAuth bearer token is always going to look the same, but we have repeated req.Header.Set("Authorization", "Bearer ...") calls.

Unifying these makes it easier for campaigns to add support for generic user tokens: rather than having to implement code host specific switches in the campaigns and repo-updater packges, we can instead implement these once in internal/extsvc and use the generic types in the relevant campaigns tables, which is a significant win.

I've elected to only update github, gitlab, and bitbucketserver, mostly because (a) that's what we need for campaigns, and (b) there are diminishing returns after that, since the other service types only support one method of authentication, and that method isn't always standard.

Implementation wise, there isn't much of note here. OAuthBearerToken probably looks a touch overengineered having a struct with a field right now instead of just being a type alias for string, but this pays off in the next PR.

This is part of #14989.

Created by: codecov[bot]

Codecov Report

Merging #15077 into main will increase coverage by 0.04%. The diff coverage is 63.41%.

@@            Coverage Diff             @@
##             main   #15077      +/-   ##
==========================================
+ Coverage   52.56%   52.60%   +0.04%     
==========================================
  Files        1610     1615       +5     
  Lines       80458    80501      +43     
  Branches     7110     7001     -109     
==========================================
+ Hits        42293    42350      +57     
+ Misses      34338    34325      -13     
+ Partials     3827     3826       -1     

Flag	Coverage Δ
#go	52.60% <63.41%> (+0.07%)
#integration	30.36% <ø> (-0.13%)
#storybook	26.12% <ø> (ø)
#typescript	52.62% <ø> (-0.04%)
#unit	34.29% <ø> (ø)

Impacted Files	Coverage Δ
...terprise/internal/codeintel/httpapi/auth_github.go	0.00% <0.00%> (ø)
internal/authz/github/client.go	42.85% <0.00%> (-7.15%)
internal/extsvc/gitlab/client.go	34.34% <0.00%> (+2.26%)
internal/extsvc/bitbucketserver/client.go	54.35% <65.30%> (+5.44%)
internal/extsvc/github/client.go	67.12% <80.00%> (-0.48%)
cmd/repo-updater/repos/github.go	64.08% <100.00%> (+0.10%)
...nterprise/cmd/frontend/auth/githuboauth/session.go	75.90% <100.00%> (ø)
internal/authz/bitbucketserver/provider.go	76.99% <100.00%> (-1.39%)
internal/authz/github/github.go	72.00% <100.00%> (ø)
internal/extsvc/auth/basic.go	100.00% <100.00%> (ø)
... and 18 more

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff 34dc5ac868a26c6807c7559d72609f9f4b5f7a13...2c81fe56ece800aac5bc4ba8accc305bd05534d8.

Notify	File(s)
@efritz	enterprise/internal/codeintel/httpapi/auth_github.go
@nicksnyder	internal/extsvc/CODENOTIFY internal/extsvc/auth/CODENOTIFY

Created by: mrnugget

(Tagged @unknwon for review here since he worked on authz/authn a lot in the past)

Created by: unknwon

(Tagged @unknwon for review here since he worked on authz/authn a lot in the past)

For the record, "past" is still the "present"