Implement LocalStructuredProperty

[takashi8]

PR for implementation of LocalStructuredProperty.

Discussion

I've found a different behavior between this PR and google.appengine.ext.ndb implementation.
LocalStructuredProperty inherits BlobProperty and use pb.SerializePartialToString() in Property._to_base_type both in google.appengine.ext and this.
But when I see stored properties in cloud console, one with google.appengine.ext is stored as an embedded entity value, while this implementation results to a blob value.

Maybe _db_set_uncompressed_meaning is the cause? I haven't found the exact reason.
If this difference matters, I'll dig in more.

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[takashi8]

Signed CLA.

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[andrewsg]

Thank you for your kind contribution, and especially thank you for signing the CLA and including tests. This PR touches some code in active development, so we may need a few days to integrate it and avoid conflicts.

[cguardia]

Thanks a lot, and sorry for the late response. I think it's OK that it is stored as a blob value.

[cguardia]

Looks good. I'll accept the PR as is and add system tests later on.