feat(oauth): use oauth device flow to authenticate with predefined src-cli OAuth client

[burmudar]

This adds the flag --oauth to login command which then starts the OAuth device authentication flow. gh does the same flow when you authenticate from the cli with gh auth login.

• add package internal/oauth
• discover oauth configuration via path .well-known/openid-configuration
• add client to handle flow, in particular to discover the endpoints it should use and ultimately poll for the token once the user as authorized the application.

In #1228 we add a keyring to store the oauth credentials.

Important

A Follow up will remove the --oauth flag and make it dynamically decide to start the flow or not based on whether the access token is set.

Test plan

• Unit tests

go test -v ./internal/oauthdevice/...
=== RUN   TestDiscover_Success
--- PASS: TestDiscover_Success (0.00s)
=== RUN   TestDiscover_Caching
--- PASS: TestDiscover_Caching (0.00s)
=== RUN   TestDiscover_Error
--- PASS: TestDiscover_Error (0.00s)
=== RUN   TestStart_Success
--- PASS: TestStart_Success (0.00s)
=== RUN   TestStart_WithScopes
--- PASS: TestStart_WithScopes (0.00s)
=== RUN   TestStart_Error
--- PASS: TestStart_Error (0.00s)
=== RUN   TestStart_NoDeviceEndpoint
--- PASS: TestStart_NoDeviceEndpoint (0.00s)
=== RUN   TestPoll_Success
--- PASS: TestPoll_Success (0.00s)
=== RUN   TestPoll_AuthorizationPending
--- PASS: TestPoll_AuthorizationPending (0.00s)
=== RUN   TestPoll_SlowDown
--- PASS: TestPoll_SlowDown (0.00s)
=== RUN   TestPoll_ExpiredToken
--- PASS: TestPoll_ExpiredToken (0.00s)
=== RUN   TestPoll_AccessDenied
--- PASS: TestPoll_AccessDenied (0.00s)
=== RUN   TestPoll_Timeout
--- PASS: TestPoll_Timeout (0.00s)
=== RUN   TestPoll_ContextCancellation
--- PASS: TestPoll_ContextCancellation (0.00s)
PASS
ok      github.com/sourcegraph/src-cli/internal/oauthdevice     0.625s

• Tested manually against my local SG

Amp thread

[burmudar]

This change is part of the following stack:

• feat(oauth): use oauth device flow to authenticate with predefined src-cli OAuth client  #1223 ◀
  • feat(oauth): Add refresh to oauthdevice.Client #1227
    • feat(oauth): Use keyring to store oauth token #1228

_{Change managed by git-spice.}