feat: custom extension support simply pass in code

[ImSingee]

The extension is a good feature. But now we can only use some "built-in" extension.

In this PR, I extend the semantics for the Extensions config so that users can also write code directly to use custom extensions.

[codablock]

I wonder if it wouldn't be better to have some generic code for Environment initialisation and then use that to actually modify the environment to have custom filters/extensions. For example:

j2.RenderString("test - {{ test_var1 | minus(-2) }}", WithInitEnvironment(`
def add(a, b):
    return int(a) + int(b)

def init_environment(environment):
    from jinja2.ext import Extension
    class DemoExtension(Extension):
        def __init__(self, environment):
            super().__init__(environment)
            environment.filters['minus'] = self.minus
            
        @staticmethod
        def minus(a, b):
            return int(a) - int(b)
    environment.add_extension(DemoExtension)
    environment.filters["add"] = add
`))

This would allow arbitrary initialisation of the environment. Based on that, you could then modify WithFilter and WithExtension to simply call WithInitEnvironment but with pre-generated python code, so that these functions are just wrappers/helpers around WithInitEnvironment.

build_env would then simply exec all the init_environments entries and look for the init_environment method and just call it.