Support FLOAT32 type

[apstndb]

This PR adds FLOAT32 type support.

spanner> SELECT CAST(1 AS FLOAT32), ARRAY<FLOAT32>[1];
+----------+------------+
|          |            |
+----------+------------+
| 1.000000 | [1.000000] |
+----------+------------+
1 rows in set (3.17 msecs)

fixes #175

[apstndb]

Some dependencies of new google-cloud-go (go.opentelemetry.io/otel) require the cmp and slices packages introduced in Go 1.21.

Error: ../../../go/pkg/mod/go.opentelemetry.io/otel@v1.28.0/attribute/set.go:7:2: package cmp is not in GOROOT (/opt/hostedtoolcache/go/1.19.13/x64/src/cmp)
Error: ../../../go/pkg/mod/go.opentelemetry.io/otel@v1.28.0/attribute/set.go:10:2: package slices is not in GOROOT (/opt/hostedtoolcache/go/1.19.13/x64/src/slices)

Sooner or later spanner-cli will require Go 1.21.

[yfuruyama]

LGTM. Thanks for quickly supporting this!