Use keyword-only parameters for backend optimization functions + do not use default value for optimization domain

[bqth29]

💬 Pull Request Description

⚠️ This PR contains breaking changes. See at the bottom of this description for more information.

Some parameters of the optimization-related functions of backend classes were using keyword-only parameters in a way that seemed quite arbitrary. This has been homogenized by putting all parameters as keyword-only, expect the optimization domain (domain) if applicable.

The domain parameter also no longer has a default value (for minimize, maximize and optimize functions) to encourage users explicitely defining the optimization domain of their choice to avoid confusion.

✔️ Check list

Before you open the pull request, make sure the following requirements are met.

• [x] The code matches the styling rules
• [x] The new code is covered by relevant tests
• [x] Documentation was added

🚀 New features

None

🐞 Bug fixes

None

📣 Supplementary information

Defining these parameters as keyword-only may have introduced breaking changes for users directly calling the impacted backend classes using a version <= 1.2.1.

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (6194a9e) to head (3137751).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #53 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 36 36 Lines 1600 1600 ========================================= Hits 1600 1600 ```

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