feat: support default workspace

[liu-hm19]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR supports the use of the default workspace (default.yaml), with which the Kusion CLI will pick up when the specified workspace is not found.

Which issue(s) this PR fixes:

Fixes #867

Special notes for your reviewer:

Does this PR introduce a user-facing change?

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

[coveralls]

Pull Request Test Coverage Report for Build 8166188967

Details

• 0 of 8 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 73.214%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/build/util.go	0	8	0.0%
<!--	Total:	0	8

Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/watch.go	1	85.93%
<!--	Total:	1

Totals
Change from base Build 8153437294:	-0.06%
Covered Lines:	6855
Relevant Lines:	9363

---

💛 - Coveralls