Spring Cloud Spanner - Support "Set" Type

[ps882]

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See
also CONTRIBUTING.md
.

Is your feature request related to a problem? Please describe.
I have a requirement where i need to store distinct values (labels) in my table but currently Spanner only allows "List" and not "Set". Which means i have to do the validation that distinct items are stored, on my own and then save the values in SpannerRepository.

An interesting observation was, when i define the field as "Set" and then call save() it gets saved. But when i try to read it gives me casting error that it cannot cast from List to Set. Which means SpannerRepository takes the Set field and saves it as List because that's what it supports

Describe the solution you'd like
Spanner Spring supports Set type so that i can define my pojo label field as "Set"

Describe alternatives you've considered
Ensuring the labels are unique on my own.

Additional context
Add any other context or screenshots about the feature request here.

https://googlecloudplatform.github.io/spring-cloud-gcp/3.2.1/reference/html/spanner.html#_supported_types

Thanks!