feat: add external secret store api and interface

[adohe]

What type of PR is this?

/kind feature

What this PR does / why we need it:

add external secret store api and interface

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 6952793517

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 75.779%

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Files with Coverage Reduction	New Missed Lines	%
pkg/engine/operation/parser/parser.go	3	75.61%
pkg/engine/operation/parser/spec_parser.go	4	56.1%

Totals
Change from base Build 6952573998:	-0.05%
Covered Lines:	6958
Relevant Lines:	9182

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💛 - Coveralls

[SparkYuan]

lgtm