Spanner: remove Cloud Storage depedency

[saturnism]

Environment details

1. Specify the API at the beginning of the title (for example, "BigQuery: ...")
   General, Core, and Other are also allowed as types
2. OS type and version: OSX
3. Java version: java 8
4. google-cloud-java version(s): google-cloud-spanner-jdbc:1.7.0

Steps to reproduce

1. Add dependnecy to google-cloud-spanner-jdbc:1.7.0
2. See the dependency tree, e.g. mvn dependency:tree -Dverbose

Code example

N/A

Stack trace

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External references such as API reference guides used

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Any additional information below

Spanner JDBC driver pulls in cloud storage dependency. This seems excess for a JDBC driver.

According to @olavloite this is used for potentially fetching credentials from GCS.

Master branch currently has the dependency marked as scope provided. However, the semantic of provided scope doesn't seem to match the intended meaning.

1. Dependency should be marked as optional (<optional>true</optional>)
2. Consider removing this dependency (and functionality) altogether. It's not documented, nor is it a JDBC driver's responsibility to fetch credentials from a remote storage.

[saturnism]

/cc @skuruppu