Use abstractions packages published on nuget

[Yury-Fridlyand]

Dependency:

• Release .net client abstractions opensearch-net-abstractions#16

Once abstractions release, we should update references to use nuget packages instead of local ones.

[MaxKsyunz]

For context, this will allow us to remove some binaries from this repo.

The packages from opensearch-project/opensearch-net-abstractions are used in integration tests. At the moment, they are part of the repo.

[wbeckler]

Update: the next .net client release is not expected to rely on abstractions as a separate nuget package.

[Yury-Fridlyand]

Not relevant since #92 and #86.