Refactor GetConverter and handle LowCardinality() type

[adamyeats]

Fixes #805. This PR does a few things:

• Handles LowCardinality() data type correctly and adds tests
• Adds a converter for String to handle the recursion in GetConverter correctly (is there a better way to do this? i guess there was a reason why there wasn't a converter there for String already, but adding one does allow us to do some recursiveness here)
• Modifies defaultConvert to handle strings and add better handling for pointers and dereferencing them
• Refactors GetConverter to make it a bit nicer (?)
• Uses reflect.PointerTo instead of reflect.PtrTo, which is now deprecated

[github-actions]

Use the following command to run this PR with Docker at http://localhost:3000:

docker run --rm -p 3000:3000 grafana/plugin-builds:0d457e36bd31428776d1f2716df7a73f6aaff5a7pre

[aangelisc]

LGTM! Great stuff @adamyeats 👍