Allow `AutoSyncReadInterval` and `AutoSyncWriteInterval` to be set after tag is Initialized

[timyhac]

Most tag attributes are only used during the creation of the tag object, but some can be configured afterwards:

1. read_cache_ms
2. debug_level
3. auto_sync_read_ms
4. auto_sync_write_ms

Currently, libplctag.NET wrapper allows 1 and 2 to be set but prevents 3 andd 4 by throwing an exception.

Having the ability to set these at runtime can be useful for interactive applications that need to change the update frequency.

Notes:

• It may be preferable to just allow all attributes to be configured after Initialize, and let the core library make the determination of whether this is sensible or not.
• These two tag attributes are not shown in the core library documentation in the tag-internal-attribute-functions section.

[kyle-github]

Oops. They are documented now! Thanks for noticing they were missing!

I should come up with something a bit more coherent for how attributes are described so that it is clear which ones can be changed at runtime.