Different RLE Encoding Specification

In the parquet-format specification https://github.com/Parquet/parquet-format/blob/master/Encodings.md#run-length-encoding--bit-packing-hybrid-rle--3
is written that the RLE encoding starts with
```
rle-bit-packed-hybrid:
length := length of the in bytes stored as 4 bytes little endian
```

while in the cpp implementation there is this description https://github.com/apache/parquet-cpp/blob/master/src/parquet/util/rle-encoding.h#L42 and the implementation seems to follow that specification
which does not include the initial
https://github.com/apache/parquet-cpp/blob/master/src/parquet/util/rle-encoding.h#L272

So which one is the correct? seems that the parquet-format is the wrong one.
DataPage.definitionLevels uses RLE and none of the example format files seem to have that initial

Also the use of both names `literal` and `bit-encoding` is confusing.

**Reporter**: [Fabrizio Milo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=fmilo)

_{**Note**: *This issue was originally created as [PARQUET-575](https://issues.apache.org/jira/browse/PARQUET-575). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Confronta la descrizione di RLE/bit-packing in Encodings.md con l’implementazione referenziata in parquet-cpp rle-encoding.h, inclusi l’uso del livello di definizione di DataPage e i file di esempio del formato. Determina quale framing e quale terminologia siano autorevoli, quindi aggiorna la specifica e chiarisci i nomi una volta risolta la discrepanza.

Scritto dal modello di indicizzazione a partire dal testo della issue.