feat: add correlation mode to template_correlation

[zhukgleb]

Added the ability to select a correlation method. The direct method is often slower; on weaker computers, calculating the spectrum offset can take hours. The "direct" flag in the scipy correlation source code refers to direct correlation using numpy. Forward compatibility is enabled by default.

[rosteen]

Thanks for the contribution, it will be nice to provide this option! I made a couple small suggestions, if you don't have any objections I'll commit those and approve.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (368cfbd) 70.73% compared to head (98428f4) 70.87%.
Report is 11 commits behind head on main.

Files	Patch %	Lines
specutils/analysis/correlation.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1114      +/-   ##
==========================================
+ Coverage   70.73%   70.87%   +0.14%     
==========================================
  Files          64       64              
  Lines        4483     4498      +15     
==========================================
+ Hits         3171     3188      +17     
+ Misses       1312     1310       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.