feat(spanner): add support for @ReadOnlyProperty

[bradperkins-gh]

This change ensures that fields annotated with @ReadOnlyProperty are excluded from mutations while remaining readable.

Sample entity with a @ReadOnlyProperty field:

@Table(name = "custom_test_table")
private static class TestEntity {
   @PrimaryKey(keyOrder = 1)
   String id;
   
   @Column(name = "custom_col")
   String something;

   @NotMapped
   String notMapped;

   @ReadOnlyProperty
   String readOnly;
 }

The field String readOnly will be populated on queries, but will not be applied to mutations. The mutation behavior is the same as fields annotationed with '@NotMapped'.

Fixes #3753
Fixes #3992

[bradperkins-gh]

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