Refactor: Move parquet metadata parsing code into its own module (#8436)

# Which issue does this PR close?

- Part of #8000
- Prep PR for #8340, to make it
easier to review

Note while this is a large (in line count) code change, it should be
relatively easy to review as it is just moving code around

# Rationale for this change

In #8340 I am trying to split the
"IO" from the "where is the metadata in the file" from the "decode
thrift into Rust structures" logic. The first part of this is simply to
move the code that handles the "decode thrift into Rust structures" into
its own module.


# What changes are included in this PR?

1. Move most of the "parse thrift bytes into rust structure" code from
`parquet/src/file/metadata/mod.rs ` to
`parquet/src/file/metadata/parser.rs`

# Are these changes tested?

yes, by CI


# Are there any user-facing changes?

No, this is entirely internal reorganization

---------

Co-authored-by: Matthijs Brobbel <m1brobbel@gmail.com>

Author: alamb

parquet/src/file/metadata/mod.rs

@@ -40,7 +40,7 @@
 //! metadata into parquet files. To work with metadata directly,
 //! the following APIs are available:
 //!
-//! * [`ParquetMetaDataReader`] for reading from a reader for I/O
+//! * [`ParquetMetaDataReader`] for reading metadata from an I/O source (sync and async)
 //! * [`ParquetMetaDataPushDecoder`] for decoding from bytes without I/O
 //! * [`ParquetMetaDataWriter`] for writing.
 //!
@@ -91,6 +91,7 @@
 //!                         * Same name, different struct
 //! ```
 mod memory;
+mod parser;
 mod push_decoder;
 pub(crate) mod reader;
 mod writer;
@@ -195,10 +196,10 @@ impl ParquetMetaData {
         ParquetMetaData {
             file_metadata,
             row_groups,
-            #[cfg(feature = "encryption")]
-            file_decryptor: None,
             column_index: None,
             offset_index: None,
+            #[cfg(feature = "encryption")]
+            file_decryptor: None,
         }
     }