postgres/io: pre-allocate buffer capacity and use extend_from_slice i…

[Aditya-9-6]

Summary

This PR optimizes put_length_prefixed in sqlx-postgres/src/io/buf_mut.rs.

Rationale & Performance Benefit

Nearly all Postgres wire protocol messages sent to the database are length-prefixed. put_length_prefixed reserves the 4-byte length header at the beginning of each message.

Currently, it calls self.extend(&[0; 4]), which iterates over array elements and may trigger intermediate re-allocations if capacity is tight.

Optimizations made:

• Pre-allocates space upfront with self.reserve(4).
• Uses zero-copy self.extend_from_slice(&[0; 4]) slice copy instead of array iterator extension.
• Streamlines database wire protocol encoding for high-throughput query pipelines.