✨ feat(bs.spline): add linear spline type

[aksiome]

Description

Related Issues

• close #417

Additional Notes

Tasks to complete before merging

• [x] I agree to release my contribution under the MPL v2 License.
• [x] My pull request is associated with an existing issue.
• [x] I have updated the changelog to reflect my contribution.
• [x] If this pull request adds or modifies a feature:
  • [x] I have documented my changes in the /docs folder.
  • [x] I have updated the metadata of the feature. See feature metadata.
  • [x] I have thoroughly tested my changes. See testing guidelines.