Add support for --stdin-filename for nested config

[keraion]

PR sqlfluff/sqlfluff#5805 added pulling in configurations from a filename when stdin was used as a source. This will allow nested configurations to be pulled in when using the content of the existing, but dirty file.

• Added executable version checking to dynamically enable this feature on supported versions of sqlfluff (the release after 3.0.5).
• Added version checking for differences between 2.3.5 and 3.0.0.
• fixes sub configs .sqlfluff not recognized #51

[RobertOstermann]

@keraion Thank you for the pull requests! I will hopefully release these changes in v3.1.0 soon.