Allow setting custom initial value for map observers via with_initial

[mzfr]

Fixes #2447

As discussed here(#2447 (comment)) with @addisoncrump.

[domenukk]

Thank you. Can you please fix clippy?

error: missing `#[must_use]` attribute on a method returning `Self`
   --> libafl/src/observers/map/mod.rs:671:5
    |
671 | /     pub fn with_initial(mut self, initial: T) -> Self {
672 | |         self.initial = initial;
673 | |         self
674 | |     }
    | |_____^

Otherwise looks good :)