SQLite support

[DanCech]

This PR adds support for using SQLite as a backend database.

Description

The SQLite storage implementation is largely based on the existing MySQL backend, with some slight differences:

• migration 005 updated to issue separate ALTER TABLE statements for each column, as SQLite does not support multi-column ALTER statements
• date-related SQL updated to use SQLite syntax
• retry mechanism added for writes to avoid SQLITE_BUSY errors (this may not actually be needed with the busy_timeout setting, but is a backup in case the database layer does return SQLITE_BUSY when heavily loaded).
• error handling for SQLite unique constraint errors

It uses the cgo-free https://modernc.org/sqlite driver.

Unless otherwise specified, it will use the WAL journal mode and specify a busy timeout of 500ms

[jon-whit]

@DanCech I changed the base branch this PR is targeting because we're devising a strategy for how to handle community storage contributions without taking on the burden of the storage implementation longer term. We want to promote development, but the maintainer team can't practically support all of them and offer quality implementations if there are a lot of them we're having to maintain.

Consequently, we're thinking of an openfga/openfga-contrib repository structure and a plugin based mechanism that will allow the OpenFGA community to run OpenFGA with externally contributed storage implementations. We likely won't merge this into main and include it in an official release until we have a more well-defined design for that plugin ecosystem, and in the (hopefully near) future the SQLite implementation you provided herein will be one of the first community sponsored storage implementations in the contrib repo. See https://github.com/openfga/openfga-contrib/blob/main/storage/sqlite/sqlite.go

[DanCech]

Thanks for the feedback @jon-whit , it looks like the base change has created quite a few conflicts. Is there anything I can do at this stage to help move things forward?