Use `true` instead of `1` as default value for `is_compressed bool` field

I noticed that the default value for the optional boolean `is_compressed` field of the `DataPageHeaderV2` struct has a default value of `1`. According to the Thrift docs a boolean value is either `true` or `false`. 

This currently works because the Apache Thrift compiler internally handles bools as ints:

• https://github.com/apache/thrift/blob/3880a09565a9a1dad028b3679746eafac268c819/compiler/cpp/src/thrift/thriftl.ll#L208-L209
• https://github.com/apache/thrift/blob/3880a09565a9a1dad028b3679746eafac268c819/compiler/cpp/src/thrift/main.cc#L748

It may however not work with other Thrift compilers that are more strict about this.

Based on the docs and a test (https://github.com/apache/thrift/blob/3880a09565a9a1dad028b3679746eafac268c819/test/ThriftTest.thrift#L406) in the Thrift repository it seems that using `true` here is the correct way of defining a default for an optional bool field.