support kusion preview --ouput json

[healthjyk]

What type of PR is this?

feature

What this PR does / why we need it:

when using cmd kusion preview, there is no way to get the pure preview result, cause there is prompt. Support --output json to print the preview result without any unnecessary prompt when succeeded.

Which issue(s) this PR fixes:

Fixes #265

Special notes for your reviewer:

Does this PR introduce a user-facing change?

the output is json marshal of struct pkg/engine/operation/models.Changes

Additional documentation e.g., design docs, usage docs, etc.:

None

[coveralls]

Pull Request Test Coverage Report for Build 4988781634

• 16 of 28 (57.14%) changed or added relevant lines in 3 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 71.734%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/engine/operation/models/action.go	0	3	0.0%
pkg/cmd/preview/options.go	14	23	60.87%

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/apply/options.go	7	71.31%

Totals
Change from base Build 4955951961:	-0.1%
Covered Lines:	4870
Relevant Lines:	6789

---

💛 - Coveralls

[amyXia1994]

lgtm

[healthjyk]

lgtm

lgtm

@amyXia1994 If there is no resource in the planned spec, there will be an empty output when --output json, you may need a judgement before json unmarshal. For the case without --output json, there is a hint, but still with successful preview.

[SparkYuan]

lgtm