Allow embedding piped-key and ssh-key in config file

[nghialv]

What this PR does / why we need it:

While running Piped on CloudRun, the Piped config and all needed credentials will be stored in Google Secret Manager.
Because the configuration file itself is stored securely, we don't need to load API-key and SSH-key from other files in that case.
The fewer secrets, the easier it is to manage.
Therefore, this PR allows embedding piped-key and ssh-key into the config file.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Allow embedding piped-key and ssh-key in Piped config file

[pipecd-bot]

Code coverage for golang is 32.02%. This pull request decreases coverage by -0.04%.

File	Function	Base	Head	Diff
pkg/config/piped.go	PipedSpec.LoadPipedKey	--	0.00%	+0.00%
pkg/config/piped.go	PipedGit.LoadSSHKey	--	0.00%	+0.00%
pkg/git/ssh_config.go	AddSSHConfig	0.00%	0.00%	+0.00%

[khanhtc1202]

/lgtm

[pipecd-bot]

The following ISSUES will be created once got merged. If you want me to skip creating the issue, you can use /todo skip command.

Details

---

1. Remove this key file when Piped terminating.

https://github.com/pipe-cd/pipe/blob/f2ea6d6e8154af241be0d13b2b4d2a0729fed7aa/pkg/git/ssh_config.go#L76-L79

This was created by todo plugin since "TODO:" was found in f2ea6d6 when #2216 was merged. cc: @nghialv.

[nghialv]

/todo skip

[nakabonne]

This helps us in a debug context as well. Thank you!
/approve

[pipecd-bot]

This pull request is APPROVED by nakabonne.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.

[pipecd-bot]

Code coverage for golang is 32.01%. This pull request decreases coverage by -0.06%.

File	Function	Base	Head	Diff
pkg/config/piped.go	PipedSpec.LoadPipedKey	--	0.00%	+0.00%
pkg/config/piped.go	PipedGit.LoadSSHKey	--	0.00%	+0.00%
pkg/git/ssh_config.go	AddSSHConfig	0.00%	0.00%	+0.00%