Show username on the header

[t-kikuc]

What this PR does / why we need it:

As title.
Since avatar images are not shown on the header for OIDC users, users want to know which account they logged in to.

• GitHub: GitHub user ID

  

• OIDC: username from IdP

  

I think role is not necessary to show.

Which issue(s) this PR fixes:

Related to #5008

See https://github.com/pipe-cd/pipecd/pull/5008#issuecomment-2277603291

Does this PR introduce a user-facing change?: Users can see their username.

• How are users affected by this change: N/A
• Is this breaking change: no
• How to migrate (if breaking change): no

For Reviewers

See here to understand why subject is used. https://github.com/pipe-cd/pipecd/blob/034054133543b7ae9d57efd360e1b0a9aae7c7db/pkg/app/server/httpapi/callback.go#L92-L103

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 22.88%. Comparing base (a61c397) to head (e61eba5). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5172 +/- ## ========================================== - Coverage 22.90% 22.88% -0.03% ========================================== Files 416 416 Lines 44598 44598 ========================================== - Hits 10216 10206 -10 - Misses 33592 33601 +9 - Partials 790 791 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.