Use constants to represent index types.

[DiscoPYF]

fix #375

@tjoubert @Zyqsempai @rossmills99 What do you think of this approach?

[DiscoPYF]

Also if it can help a future soul, I spent hours wondering why IndexApiClientTestFixture.InitializeAsync was called more than once when doing right-click in IndexApiClientTest -> "Run Test(s)", creating test failures because the test collection was created multiple times. Turns out it's a known issue with Visual Studio and the workaround is to run all tests from the "Test Explorer".

xunit/xunit#2347

I'm using Microsoft Visual Studio Enterprise 2019 Version 16.11.10

[DiscoPYF]

I will push an update to resolve the conflicts.

[DiscoPYF]

Resolved conflicts.