this adds support for passing in documents to termvector and mtervector

As introduced here https://github.com/elasticsearch/elasticsearch/pull/7530

This uses an altered rest-api-spec as pushed back to to elasticsearch here:

https://github.com/elasticsearch/elasticsearch/pull/8869
https://github.com/elasticsearch/elasticsearch/pull/8868

fixes https://github.com/elasticsearch/elasticsearch-net/issues/1101