feat: implement variable set

[liu-hm19]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR implements a variable set of kusion server to manage the global variables for kusion module generator.

Which issue(s) this PR fixes:

Fixes #1334

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 13304889274

Details

• 78 of 891 (8.75%) changed or added relevant lines in 22 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.4%) to 46.415%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/server/handler/render.go	10	12	83.33%
pkg/server/route/route.go	26	32	81.25%
pkg/server/manager/variable/util.go	0	8	0.0%
pkg/server/manager/variableset/util.go	0	8	0.0%
pkg/server/middleware/message.go	0	8	0.0%
pkg/domain/entity/variable_set.go	0	11	0.0%
pkg/domain/entity/variable.go	0	14	0.0%
pkg/infra/persistence/variable_set_model.go	0	19	0.0%
pkg/infra/persistence/util.go	0	20	0.0%
pkg/infra/persistence/variable_model.go	0	23	0.0%

Totals
Change from base Build 13177852365:	-1.4%
Covered Lines:	11382
Relevant Lines:	24522

---

💛 - Coveralls