Allow Custom Feature Spaces in Dialogue

[Zethson]

Originally contributed by @grpinto.

Fixes #668 where users requested greater flexibility in specifying the feature space for the Dialogue implementation. Previously, the Dialogue function was limited to using PCA via the _pseudobulk_pca method, which prevented users from easily experimenting with or formally documenting the use of alternative feature reduction techniques.

Changes Introduced:

Refactored Feature Extraction:
The internal function call has been updated from _pseudobulk_pca to _pseudobulk_feature_space. Small adjustment to make the label flexible to the user.

Existing functionality has been preserved. For users who rely on the default PCA behavior, the changes maintain backward compatibility while offering an easy path to extend or replace the feature space as needed.
Testing & Verification:

Confirmed that the Dialogue function correctly applies the specified feature space.
Ran all existing tests to ensure no regressions in other parts of the codebase.