MSlice is made and optional dependency of mantidworkbench for conda installs

[warunawickramasingha]

Description of work

MSlice is turned into an optional dependency of mantidworkbench. However, mslice would still be installed explicitly in IDAaaS and inside standalone packages.

Fixes #39237.

To test:

1. Run build_packages_from_branch pipeline for this branch with parameters as BUILD_DEVEL=none, BUILD_PACKAGE=all, PUBLISH_TO_ANACONDA=true, ANACONDA_CHANNEL_LABEL=mslice_unstable, BRANCH_NAME=39237_making_mslice_optinal_dependancy
2. Install the resulting conda packages and standalone packages,
3. Verify that MSlice is not included in conda package
4. Verify MSlice is included in the standalone installer for Windows, Linux and MacOS
5. Verify MSlice is installed in IDAaaS

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[warunawickramasingha]

Note:
This PR cannot be tested until changes from the below PRs are merged into the main

1. Remove link between muon and curvefitting libs #39656
2. Avoid Linux standalone launching error #39670

[thomashampson]

I have created a PR to ensure mslice is always available on IDAaaS.

[thomashampson]

I have tested a linux build ina conda environment and standalone install on IDAaaS and the behaviour is as expected:

• mslice is not installed when I install mantidworkbench in a conda environment
• mslice is rpesent in the standalone installation
  I have also checked that the run export (the mslice pinning) works as expected.

I have one small comment on the clarity of the release notes.

[thomashampson]

All tested (see above), and happy with the release notes.