bugfix: Set Dataset.Spec.Mounts as a required property

[TrafalgarZZZ]

Ⅰ. Describe what this PR does

Currently, Dataset.Spec.Mounts is an optional property, allowing users to create a Dataset without mounts. This PR changes it as a required property to make CRD validation works.

For example, the following will be illegal after this PR:

apiVersion: data.fluid.io/v1alpha1
kind: Dataset
metadata:
  name: hbase
spec:

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. List the added test cases (unit test/integration test) if any, please explain if no tests are needed.

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[cheyang]

/lgtm
/approve

[cheyang]

/hold

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
8.4% Duplication

[cheyang]

/test fluid-e2e

[codecov]

Codecov Report

Merging #3240 (b44fa4d) into master (8322397) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head b44fa4d differs from pull request most recent head 6df70fc. Consider uploading reports for the commit 6df70fc to get more accurate results

@@            Coverage Diff             @@
##           master    #3240      +/-   ##
==========================================
- Coverage   65.54%   65.53%   -0.01%     
==========================================
  Files         397      397              
  Lines       23111    23111              
==========================================
- Hits        15148    15146       -2     
- Misses       6182     6183       +1     
- Partials     1781     1782       +1     

Impacted Files	Coverage Δ
api/v1alpha1/dataset_types.go	8.69% <ø> (ø)

... and 1 file with indirect coverage changes

[cheyang]

/lgtm
/approve

[fluid-e2e-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheyang

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