(feat) Support requestthreads persearch

[thomasht86]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

Why

The goal of the PR is to demonstrate how the new ServicesConfiguration can be used to define arbitrary services.xml configurations, with requestthreads persearch threads as an example, ref #895

The demonstration is added to the advanced_configurations.ipynb-notebook.

What

This pull request some minor changes to the vespa package, focusing on adding support for num_threads_per_search in rank profiles and improving XML service configuration validation. The most important changes are grouped into three themes: new features and test enhancements.

New Features:

• Added num_threads_per_search parameter to the RankProfile class, allowing customization of the number of threads per search. (vespa/package.py, vespa/templates/schema.txt) [1] [2] [3] [4] [5] [6] [7]
• Updated test_package.py to include num_threads_per_search in the rank profile tests. (tests/unit/test_package.py) [1] [2]

Test Enhancements:

• Introduced TestColbertLongServicesConfiguration class to validate XML service configurations and generate XML dynamically for testing. (tests/unit/test_configuration.py)
• Simplified XML validation in existing tests by removing redundant conversion to etree objects. (tests/unit/test_configuration.py) [1] [2]

These changes aim to improve the flexibility of rank profile configurations and ensure robust validation of service configurations.

CC: @jobergum