We have a number of pip-installable optional packages, for which there is no conda package.

Here we add a mechanism to add such packages using a pip: line
in src/environment-optional.yml and src/environment-dev.yml (#33740).

We also add lots of conda.txt files.

To test, follow the instructions in https://doc.sagemath.org/html/en/installation/conda.html#using-conda-to-provide-all-dependencies-for-the-sage-library-experimental
using src/environment-optional.yml or src/environment-dev.yml

(A use case for the pip line in src/environment-dev.yml is #34524: Pip package git_trac_command.)

CC: @tobiasdiez @dimpase @isuruf @saraedum @slel @yuan-zhou

Component: porting

Author: Matthias Koeppe

Branch/Commit: 0e4a696

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/33613