Add MTLS support.

[dompuiu]

Description

Provide a MTLS fetch method when the MTLS certificate is present on the worker.

Related Issue

Motivation and Context

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA or I'm an Adobe employee.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have made any necessary test changes and all tests pass.