Add ability to list a collection's Headers without the document contents

[vbmade2000]

Closes #193.

[vbmade2000]

Still WIP. Confused about what to implement next.

[ecton]

What do you feel is missing from the issue? Other than the unit test not verifying the output, I think this completes all the requirements in the issue?

[ecton]

Ah, after running the CI, I see this error:

no method named `headers` found for struct `connection::List` in the current scope

This is telling you that you need to add a method named headers to connection::List. As per the linked issue, this should be implemented similarly to connection::List::count

[vbmade2000]

I introduced a typo in commit message by mistake. It should be Write test for list_headers

[ecton]

All of the code looks good at first glance, but the CI is failing to compile with this error:

 error[E0308]: mismatched types
Error:    --> crates/bonsaidb-core/src/connection.rs:961:60
    |
961 |                     .list_headers::<Cl, _, _>(range, sort, limit)
    |                                                            ^^^^^ expected `usize`, found `u32`
    |
    = note: expected enum `std::option::Option<usize>`
               found enum `std::option::Option<u32>`