add clock uncertainty configuration option

[jmitrevs]

Description

Allow for changing the clock uncertainty more easily in the config

Type of change

• [x] New feature (non-breaking change which adds functionality)

Tests

Added tests to both test_keras_api and test_pytorch_api.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [x] I have added tests that prove my fix is effective or that my feature works.

[jmitrevs]

Still missing: tests, update documentation, and update other than keras parsing.

[jmitrevs]

It seems like this option is just for Vivado_HLS and Vitis_HLS. There probably is a better way of specifying it than adding it to all the backend initial configs.