Deploy the lesion segmentation model in SCT

[jcohenadad]

Now that the model is ready to be alpha-tested and used by external collaborators (eg: @leelisae), we need to port the model to SCT.

The plan is to:

• [x] Merge https://github.com/spinalcordtoolbox/spinalcordtoolbox/pull/4443
• [x] Have external collaborators install SCT's master branch

Related issue: https://github.com/spinalcordtoolbox/spinalcordtoolbox/issues/4442

[plbenveniste]

The model was successfully added to SCT v6.3. The collaborators therefore only need to install the latest release to use the model.

Closing this issue. Will reopen this issue if we get feedback from collaborators.