Control TK + GB stitching with TK + GB config flags

What?

• Add new config variables include_tk, include_gb
• Replace run_poller with new flags to control which sources contribute to the stitched set
• Stitcher reads both flags and includes relevant sources (slug conflicts preserved - GB always wins if both flags are true)
• Poller no longer checks run_poller (removed)
• Experiments Controller logs which sources build the experiments API responses
• Default to TK source if both sources are disabled
• Add better logging to include which sources are being used

Why?

Relying on run_poller configuration alone leads to no experiments being included in the experiments API responses if the poller is disabled. By switching to source-level configuration variables, we can have granular control over which experiments are included in the responses - TK-registered and/or GB-sourced.

Testing Instructions

• Toggle setting include_tk and include_gb to true, false in config.local.yaml
• When one or the other is true, those are the experiments seen in the experiments API responses while the other is excluded
• When both are true, both are included
• When neither are true, the default response/failsafe returns TK-registered experiments

Bug: T428017