[GCS]Add abstract interface of actor to GCS Client

[micafan]

Add abstract interface for reading or writing or subscribing actors to GCS Client. Through the abstract interface, GCS can be extended to support other storage systems.

Related issue number

Closes #5058

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[edoakes]

LGTM, just a few more nits

[edoakes]

Ah sorry, meant to approve....

[raulchen]

Regarding the file structure, it seems that we'll have 5 files (x_info_accessor.h, redis_x_info_accessor.h/cc, and service_based_x_info_accessor.h/cc). That's a bit too complicated. What about just put everything in 2 files x_info_accessor.h/cc?

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