pkg/spanner/client: Add optional option.ClientOption parameters to Ne…

[mskwon]

…wClient constructor

Fixes #59

WHAT

This changes the constructor signature for the wrench client so that client options can be passed to the wrapped spanner clients. As the options are passed as a variadic parameter, there is no impact on pre-existing code.

WHY

The ability to pass client options allows for the passing of option.WithGRPCConn. This makes it so that wrench will work with spannertest - an in-memory fake Cloud Spanner which can be used for unit tests.

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[kazegusuri]

Please sign the CLA.

[mskwon]

Apologies for the delay here

[mskwon]

Bump

[kazegusuri]

Thank you for making this PR. I thought the option should be inside the wrench's config struct instead of first type options because it's the Spanner specific options. I create another PR on the top of your commit.

[kazegusuri]

Ah I need to merge this PR first to use your commit because we can't use other merge options except squash and merge.