feat: add spec storage and integrate with generate cmd

[adohe]

What type of PR is this?

/kind feature

What this PR does / why we need it:

add spec storage implementation

Which issue(s) this PR fixes:

Fixes #

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 8414854393

Details

• 0 of 178 (0.0%) changed or added relevant lines in 11 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 58.448%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/cmd/generate/generator/generator.go	0	2	0.0%
pkg/backend/storages/local.go	0	3	0.0%
pkg/backend/storages/mysql.go	0	3	0.0%
pkg/backend/storages/oss.go	0	3	0.0%
pkg/backend/storages/s3.go	0	3	0.0%
pkg/backend/backend.go	0	6	0.0%
pkg/engine/spec/storages/util.go	0	9	0.0%
pkg/engine/spec/storages/local.go	0	28	0.0%
pkg/cmd/generate/generate.go	0	32	0.0%
pkg/engine/spec/storages/oss.go	0	41	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/cmd/generate/generate.go	1	0.0%
pkg/cmd/generate/generator/generator.go	1	0.0%

Totals
Change from base Build 8414798303:	-0.6%
Covered Lines:	7327
Relevant Lines:	12536

---

💛 - Coveralls

[liu-hm19]

lgtm