Unify separators in `unique_key` construction (`|` vs `_`)

[vdusek]

Description

We currently construct parts of the unique_key using the pipe (|) as a separator.
For example, a standard (extended) unique_key looks like this:

unique_key='GET|e3b0c442|e3b0c442|https://crawlee.dev/'

However, for the always_enqueue feature, we append a random alphanumeric suffix using an underscore (_) instead of a pipe:

if always_enqueue:
    unique_key = f'{unique_key}_{crypto_random_object_id()}'

Resulting in:

unique_key='GET|e3b0c442|e3b0c442|https://crawlee.dev/_7VnDVJgE3TrjbDl86'

Proposed change

Use the pipe (|) consistently as a separator throughout all unique_key constructions, including the always_enqueue case.

Note

This change affects the resulting unique_key format and may be breaking, so it should be addressed as part of the next major release (v2.0).

[vdusek]

Since this proposal changes only the unique keys of the always_enqueue request, it is not a breaking change.