feat: store multiplexer

[shizhMSFT]

What this PR does / why we need it:

This PR introduces a multiplexer for stores.
The multiplexer is intended to be used when specifical stores are required for certain registries.

Which issue(s) this PR resolves

Related to #19, #26, #30.

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

Attention: Patch coverage is 90.69767% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
store_mux.go	90.69%	8 Missing and 4 partials ⚠️

Files with missing lines	Coverage Δ
store_mux.go	90.69% <90.69%> (ø)

[shizhMSFT]

Examples for typical scenarios like mixing [OCIStore] and [RegistryStore] will be added in later PRs as it depends on #30.
One interesting use case is to use the multiplexer for special registries like localhost and those registries require client-cert auth.