Add convenience method for YaraScanner

[JSCU-CNI]

This convenience method makes it easier to use the YaraScanner with a Python defined YARA rule. This makes one off, single file plugins easier to make because one can include the YARA rule directly in Python instead of having to include the rule in a separate YARA file. I.e.:

RULE = """rule ExampleRule
{
    strings:
        $line = { 00 01 02 03 }

    condition:
        all of them
}
"""

class Plugin:
    [setup]
    
    def _generator(self, layer):
        rule = yarascan.YaraScanner.from_text(RULE)
        scanner = yarascan.YaraScanner(rules=rule)
        [...]
""""

[ikelos]

Oh, ruff spotted that formatted_rule now isn't defined, we can just use rule. 5:)

[ikelos]

Also, the type hinting uses yara which may not exist if yara_x is the only installed instance. The previous methods avoided it by just not type hinting, but if you want to do it correctly, I guess assign the type to a variable after the import has been made, and then use that type in the signature...