Introduce index_feature for all index

[LHT129]

The characteristics of each index are often different. Users often want to know the specific characteristics of the index, such as whether it supports adding data from empty index, whether it supports deletion, whether it supports concurrent search and insertion, etc.

In fact, the internal functions of each index should also be implemented according to the characteristic granularity, which is also convenient for testing and verification, and is more friendly to developers.

Therefore, we provide an enum of all characteristics in the external interface, and at the same time provide interface methods for printing the characteristics of the current index for a specific index

enum class IndexFeature {
     SUPPORT_ADD_WITHOUT_TRAIN = 1,
     SUPPOET_DELETE_BY_ID = 2,
     ...
}

std::vector<Feature> Index::GetFeatures();
bool Index::CheckFeature(IndexFeature feature);

the related pull requests:
#199
#210

todo:

• add feature object
• introduce it to all kinds of index
• use the feature to check and reconstruct some test

[wxyucs]

sounds good, will it also available in the pyvsag?