fix: export showcase_v1beta1.yaml from BUILD.bazel to support external GAPIC generation

[burkedavison]

By exporting *.yaml from this BUILD file, gapic-generator-java repo will be able to directly reference the showcase_v1beta1.yaml when invoking its java_gapic_library target.

java_gapic_library(
    name = "showcase_java_gapic",
    # ...
    service_yaml = "@com_google_gapic_showcase//schema/google/showcase/v1beta1:showcase_v1beta1.yaml"
    # ...
)

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