Skipping of Structured Fields in AfpInputStream without Parsing to SF-Objects

[moritzfl]

When using AfpInputStream to get information about certain Structured Fields within an AFP-File, not all Structured Fields might need to be interpreted completely.

For performance reasons it might then be useful to skip Structured Fields without parsing them to SF objects. This PR adds this functionality to AfpInputStream.

[michaelknigge]

Hi @yan74,

any chance this get's merged? Or is this project dead? No release since Nov 2020 ....

Thanks, Michael