[Improvement Suggestion][sqlfluff] Allow other config file other than `.sqlfluff`

`sqlfluff` looks for the [list of these files](https://docs.sqlfluff.com/en/stable/configuration.html#configuration-files) here automatically and should not require a hardcoded .sqlfluff to be looked up in the [linter](https://github.com/dense-analysis/ale/blob/29e5ffe35a2fa1ca5ba19eebfe7cdd0191c16e39/ale_linters/sql/sqlfluff.vim#L22) and in the [fixer](https://github.com/dense-analysis/ale/blob/29e5ffe35a2fa1ca5ba19eebfe7cdd0191c16e39/autoload/ale/fixers/sqlfluff.vim#L13). Users of other config file and in my case pyproject.toml have no choice but to migrate to `.sqlfluff`

I want to suggest that we remove the constraint on allowing only `.sqlfluff` file as config file and let `sqlfluff` search for it.