Pgvector

- Fixed error with `ANALYZE` and vectors with different dimensions

If upgrading with Postgres 12 or Docker, see [these notes](https://github.com/pgvector/pgvector#060).

- Added support for parallel index builds for HNSW
- Added validation for GUC parameters
- Changed storage for vector from `extended` to `external`
- Improved performance of HNSW
- Reduced memory usage for HNSW index builds
- Reduced WAL generation for HNSW index builds
- Fixed error with logical replication
- Fixed `invalid memory alloc request size` error with HNSW index builds
- Moved Docker image to `pgvector` org
- Added Docker tags for each supported version of Postgres
- Dropped support for Postgres 11

- Improved performance of HNSW index builds
- Added check for MVCC-compliant snapshot for index scans

- Added HNSW index type
- Added support for parallel index builds for IVFFlat
- Added `l1_distance` function
- Added element-wise multiplication for vectors
- Added `sum` aggregate
- Improved performance of distance functions
- Fixed out of range results for cosine distance
- Fixed results for NULL and NaN distances for IVFFlat

- Improved error message for malformed vector literal
- Fixed segmentation fault with text input
- Fixed consecutive delimiters with text input

- Improved cost estimation
- Improved support for spaces with text input
- Fixed infinite and NaN values with binary input
- Fixed infinite values with vector addition and subtraction
- Fixed infinite values with list centers
- Fixed compilation error when `float8` is pass by reference
- Fixed compilation error on PowerPC
- Fixed segmentation fault with index creation on i386