Implement extraction of the data part of data portion in WAL segments

[sh-soltanpour]

Is your feature request related to a problem? Please describe.
As the last part of #296, we need to parse the data part of the data portion from the image below. To do this, we probably need information from both header parts (#299 and #300).

Describe the solution you'd like
The data part consists of one or more block data and one main data data structure, and the whole data can be categorized into one of the two data structures: BackupBlock and NonBackupBlock. Based on the information from the header, the solution must recognize the correct type of data structure to be used and then parse the data accordingly.

Additional information

Any additional information you can provide, like an overall design description