feat: PostgreSQL compatibility

[adamantike]

This change introduces PostgreSQL compatibility, by implementing the following changes:

• Use JSONB instead of JSON for PostgreSQL databases. This is achieved by creating a custom CustomJSON type that uses JSONB on PostgreSQL and JSON on other databases, leveraging the variant mechanism of SQLAlchemy.
• Add is_postgresql function to check if the current database is PostgreSQL. Commonly used for migrations, but can be used to determine how to build queries based on the database engine.
• Add json_array_contains_value function to check if a JSON array includes a specific value. This function is needed as it's engine-specific.

Support for PostgreSQL is on a best-effort basis, and it relies on the community for continued support and testing. The project's main database is MariaDB.

Closes #667.

[github-actions]

Test Results

91 tests  ±0   91 ✅ ±0   27s ⏱️ -2s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 3f4a36a. ± Comparison against base commit 82e718d.

[gantoine]

surprised sqlalchemy doesn't offer a utility for this

[adamantike]

I haven't found a way for MySQL to provide a similar functionality to PostgreSQL's ? operator, which checks if a value exists in a JSONB array, or as a key for a JSONB object.

Maybe that's why it's not simple to provide this in SQLAlchemy, unless without knowing which kind of value the JSON column will contain:

SELECT JSON_CONTAINS('{"a": 1, "b": 2, "c": {"d": 4}}', '"a"', '$');  -- FALSE
SELECT JSON_CONTAINS('["a", "b", "c"]', '"a"', '$');  -- TRUE

SELECT JSON_CONTAINS_PATH('{"a": 1, "b": 2, "c": {"d": 4}}', 'one', '$.a');  -- TRUE
SELECT JSON_CONTAINS_PATH('["a", "b", "c"]', 'one', '$.a');  -- FALSE

If someone finds a way to do that, I think that could be contributed to SQLAlchemy, as a Comparator.has_key function for the mysql.JSON dialect type, similar to the PostgreSQL implementation.