Add table quantization option for pgvector

[lucagiac81]

This PR adds a table quantization option for pgvector.

• The table_quantization_type option controls quantization of data as it is loaded into the database.
• This is in addition to the existing quantization_type option which controls quantization of data in an index.

Quantizing data as it is loaded enables storage savings if the original float vectors do not need to be preserved for other purposes. For example, halfvec quantization could be used in both the table and index, reducing the size of the table and eliminating the need for conversions.

table_quantization_type supports the same quantization options as quantization type: none, halfvec, bit.
Most combinations of the two options are supported, with some exceptions.

• If table_quantization_type = bit, then quantization type is forced to bit as well, as it does not make sense to convert bit vectors in the table to vector or halfvec in an index.
• Reranking is only supported if quantization_type = bit and table_quantization_type is vector or halfvec.

[sre-ci-robot]

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This pull-request has been approved by: alwayslove2013, lucagiac81
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