Fix cache service name

[maggie-lou]

I realized that in the generated code, it automatically appends Server to the service name. So with the previous name, it was named CacheServiceServer

[luluz66]

It seams like we have examples with Service suffix. MetadataService and AuthService. Another option is to name the service Api, since the package name indicates the service name. For example raft_service.Api and ping.Api. I felt API might make sense, so cache_service.Api vs cache_service.Cache

[maggie-lou]

It seams like we have examples with Service suffix. MetadataService and AuthService. Another option is to name the service Api, since the package name indicates the service name. For example raft_service.Api and ping.Api. I felt API might make sense, so cache_service.Api vs cache_service.Cache

With this PR, it will look like cache_service.CacheServer.
Previously, it would look like cache_service.CacheServiceServer

We often abbreviate proto package names, so I don't like naming the service API because it could create ambiguity. Ex. cspb.APIServer

Another option is to move the cache service definition into cache.proto. So it would be cache.CacheServer

Honestly I don't have a strong preference here, but cache_service.CacheServer is still probably my choice

[vanja-p]

I like cache_service.CacheServer. I don't really like having API or Service in the name, since it's redundant: service CacheService. The remote execution services are good examples: ActionCache, Execution, ContentAddressableStorage.

Another reason I don't like API is that it irrationally bothers me when I see it written as Api because of camelcase.