Add optional username and email fields to repo for commits

[Warashi]

What this PR does:

This PR adds username and email fields to the repo struct.

Why we need it:

We need these fields to do the git commit.
Originally, these fields are set when creating the new repo in the git client's Clone method.
When I implemented the CopyToModify method, I forgot to set these fields.

ref;

pipecd/pkg/git/client.go

Lines 239 to 244 in f7e3bbc

	if c.username != "" || c.email != "" {
	if err := r.setUser(ctx, c.username, c.email); err != nil {
	return nil, fmt.Errorf("failed to set user: %v", err)
	}
	}

Which issue(s) this PR fixes:

Without this PR, the piped cannot handle the GIT_UPDATE event successfully.

How to reproduce the issue

Remove global git config, run piped, and send the event with pipectl.

You will see the log like below.

{"severity":"ERROR","eventTime":1736988484.684635,"logger":"piped.piped.event-watcher","caller":"eventwatcher/eventwatcher.go:403","message":"failed to commit outdated files","serviceContext":{"service":"piped.piped","version":"unspecified"},"error":"failed to commit, branch: main, error: err: exit status 128, out: Author identity unknown\n\n*** Please tell me who you are.\n\nRun\n\n  git config --global user.email \"[email protected]\"\n  git config --global user.name \"Your Name\"\n\nto set your account's default identity.\nOmit --global to set the identity only in this repository.\n\nfatal: unable to auto-detect email address (got 'warashi@nixos.(none)')\n","stacktrace":"github.com/pipe-cd/pipecd/pkg/app/piped/eventwatcher.(*watcher).execute\n\t/home/warashi/ghq/github.com/pipe-cd/pipecd/pkg/app/piped/eventwatcher/eventwatcher.go:403\ngithub.com/pipe-cd/pipecd/pkg/app/piped/eventwatcher.(*watcher).run\n\t/home/warashi/ghq/github.com/pipe-cd/pipecd/pkg/app/piped/eventwatcher/eventwatcher.go:294"}

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[ffjlabo]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 26.24%. Comparing base (58fda21) to head (38b5c92).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/git/repo.go	42.85%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5477   +/-   ##
=======================================
  Coverage   26.24%   26.24%           
=======================================
  Files         458      458           
  Lines       49364    49370    +6     
=======================================
+ Hits        12954    12956    +2     
- Misses      35383    35386    +3     
- Partials     1027     1028    +1     

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