Add 6 and 8 argument overloads to wiener_lpdf

[WardBrian]

From stan-dev/math#2822
Scalar only for now, see #1373 for partial reason

I didn't add support for the optional precision_derivatives argument both of these have in C++. These would need to be data, which we don't currently support in the machinery for our distributions. We can add them manually or rethink this later.

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here: TBD

Release notes

Exposed the new overloads of wiener_lpdf added to Stan Math by @Franzi2114

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.87%. Comparing base (8bc6ba0) to head (0ce17bf).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1411   +/-   ##
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  Coverage   89.87%   89.87%           
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  Files          63       63           
  Lines       10525    10529    +4     
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+ Hits         9459     9463    +4     
  Misses       1066     1066           

Files	Coverage Δ
src/middle/Stan_math_signatures.ml	98.02% <100.00%> (+<0.01%)	⬆️