Broaden SQLAlchemy support in llama-index-vector-stores-postgres to 1.4+

[jamesoc-eiq]

Description

llama-index-vector-stores-postgres currently has a dependency on SQLALchemy 2.0.
However, there is a large body of existing code written for SQLAlchemy 1.4.x, which is still maintained and in active use.
This PR allows llama-index-vector-stores-postgres to be used with both versions, making is vastly easier to integrate into existing codebases. It also harmonises the repo with llama-index-core which currently depends on 1.4.49.
Added input validation for schema_name to allow safe usage of format string instead of an SQL param for schema creation.

Type of Change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Re-ran existing tests, all passed with 100% pass rate, using a pgvector/pgvector:pg14 docker container as back-end:

docker run -d \
  --name pgvector_container \
  -e POSTGRES_PASSWORD=mark90 \
  -p 5432:5432 \
  pgvector/pgvector:pg14

• 100% passing tests for both SQLAlchemy 1.4.49 and 2.0.25
• I stared at the code and made sure it makes sense

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings (existing, unrelated deprecation warnings have been suppressed for clarity
• New and existing unit tests pass locally with my changes
• I ran make format; make lint to appease the lint gods