Adding file mode option to serialize() methods

[chatman]

In serialize() methods of CAGRA, Brute Force and HNSW, it would be preferable to add the ability to append to a file (ios::app). This will be useful for Lucene, where the file written to the segment contains a header, post which the serialized index could be written, by opening the file in append mode.

Also, useful for composite files (file containing multiple indexes). For this usecase, though, a deserialize() method where indexes from the same file can be loaded using offsets+length. This is not in scope for this PR, though.

In this case, the additional file_mode parameter can have 'a' for ios::app or 'w' for ios::out.

cuvsError_t cuvsBruteForceSerializeWithMode(cuvsResources_t res,
                                             const char* filename,
                                             cuvsBruteForceIndex_t index,
                                             char file_mode);

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[mythrocks]

Question: Why was this option required for nvcc?

[mythrocks]

Is the indent off by 1 here?

[cjnolet]

@chatman is this still relevant given the more recent move to serializing cagra out to Lucene HNSW format directly?

[cjnolet]

@chatman ping ^^

[chatman]

@chatman is this still relevant given the more recent move to serializing cagra out to Lucene HNSW format directly?

For CAGRA or Tiered index serialization, either this or #1085 is needed. My preference would be #1085, and in that case we can close this.

[chatman]

Closing this in favour of #1085.