Implement webhook validation for the XGBoostJob

[tenzen-y]

What this PR does / why we need it:
I implemented webhook validations for the XGBoostJob.
Additionally, I didn't add any additional validations. The traininig-operator has the same validations the same as before.

Although we potentially commonize podTemplate validations, I leave such commonize in the follow-ups.

This PR depends on #2035

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Part-of #1993

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenzen-y

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[tenzen-y]

/hold

[coveralls]

Pull Request Test Coverage Report for Build 8635149231

Details

• 105 of 248 (42.34%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 35.173%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/webhooks/webhooks.go	0	3	0.0%
pkg/webhooks/xgboost/xgboostjob_webhook.go	49	74	66.22%
pkg/webhooks/pytorch/pytorchjob_webhook.go	56	83	67.47%
pkg/cert/cert.go	0	31	0.0%
cmd/training-operator.v1/main.go	0	57	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/training-operator.v1/main.go	1	0.0%

Totals
Change from base Build 8585006097:	-0.2%
Covered Lines:	4341
Relevant Lines:	12342

---

💛 - Coveralls

[tenzen-y]

Rebased.

[tenzen-y]

This is replaced with #2052.
/close

[google-oss-prow]

@tenzen-y: Closed this PR.

In response to this:

This is replaced with #2052.
/close

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