Implement webhook validations 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
for review

[coveralls]

Pull Request Test Coverage Report for Build 8637552831

Details

• 49 of 74 (66.22%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on implement-xgboost-webhook-validation at 35.211%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/webhooks/xgboost/xgboostjob_webhook.go	49	74	66.22%

Totals
Change from base Build 8637479311:	35.2%
Covered Lines:	4350
Relevant Lines:	12354

---

💛 - Coveralls

[tenzen-y]

These are the third webhook validations. PTAL.
/assign @andreyvelich @johnugeorge

[andreyvelich]

Thanks @tenzen-y!
/lgtm
/assign @johnugeorge

[johnugeorge]

/hold cancel