Add Batch Read for Line Items

[roycornelissen]

Implements #54: batch-read API for Line Items.

This method returns a somewhat unconventional response compared to other API's: it is a dictionary of LineItemHubSpotEntity where each key is a line item ID, and each value is a LineItemHubSpotEntity. I added DeserializeDictionaryOfEntities<TKey, TEntity>() to the RequestSerializer to facilitate this.

[roycornelissen]

@nover Hi! Back with one more PR 😄 Could you have a look at this? We're using this new method in our system with a private build of this library; would be great if we could continue using it from the official build.

[nover]

The hubspot responses never cease to amuse me. Thanks for contributing 👌

[roycornelissen]

@nover will you release it? Not sure yet how the release process goes for this repo 😊

[nover]

Yes sure! It's partly a manual process where I create a tag using dotnet-version-cli and push that.