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 #5008 (comment)

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.

pipecd/pkg/app/server/httpapi/callback.go

Lines 92 to 103 in 0340541

	user, err := getUser(ctx, sso, proj, authCode)
	if err != nil {
	h.handleError(w, r, "Unable to find user", err)
	return
	}
	claims := jwt.NewClaims(
	user.Username,
	user.AvatarUrl,
	tokenTTL,
	*user.Role,
	)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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     

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[Warashi]

👍🏻

[ffjlabo]

🚀