Feature Request: Allow key field to point to functions

[Swoorup]

Description

Currently, the key field in the venndb library is limited to pointing to struct fields. We could an enhancement to allow the key field to point to functions inside the struct, providing more flexibility in key definitions and usage.

Example

#[derive(VennDB)]
#[venndb(key = "get_dynamic_key")]
struct ExampleStruct {
    #[venndb(key)]
    id: u32,
    name: String,
}

impl ExampleStruct {
    fn get_dynamic_key(&self) -> u32 {
    }
}

Benefits

Increased flexibility in defining keys for the venndb library.
Support for dynamic key generation based on custom logic.