New custom variable for tos uncertainty

[LisaBock]

Description

This pull request adds a custom cmor table entry for sea surface temperature uncertainty (tosStderr), and a test for this new entry.

Closes #2469

---

Before you get started

• [x] ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• [x] 🧪 The new functionality is relevant and scientifically sound
• [x] 🛠 This pull request has a descriptive title and labels
• [x] 🛠 Code is written according to the code quality guidelines
• [x] 🧪 and 🛠 Documentation is available
• [x] 🛠 Unit tests have been added
• [x] 🛠 Changes are backward compatible
• [x] 🛠 Any changed dependencies have been added or removed correctly
• [x] 🛠 The list of authors is up to date
• [x] 🛠 All checks below this pull request were successful

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.62%. Comparing base (60aec3e) to head (6f6b887).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2470 +/- ## ======================================= Coverage 94.62% 94.62% ======================================= Files 247 247 Lines 14057 14057 ======================================= Hits 13301 13301 Misses 756 756 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.