🏗 Add an rc-channel experiment ID to allow opt-in to the RC build of AMP

[rsimha]

We now release 4 flavors of the AMP runtime:

• Canary
  • Version prefix 00
  • Contains the latest code, and enables the experiments in canary-config.json
  • Served to 0.5% of all requests, and to those who opt in to the dev-channel experiment
• Prod
  • Version prefix 01
  • Contains one-week-old code, and enables the experiments in prod-config.json
  • Currently served to 99% of all requests (soon to be 98.5%)
• Control
  • Version prefix 02
  • Contains the same one-week-old code as Prod, and enables the experiments in prod-config.json
  • Served to 0.5% of all requests
  • Currently used to compare error rates with Canary (differences may be due to experiment config, or code, or both)
• RC
  • Version prefix 03
  • Contains the latest code, and enables the experiments in prod-config.json
  • Currently served to 0% of all requests (soon to be 0.5%), and to those who opt in to the rc-channel experiment
  • Soon to be used to compare error rates with Canary (only difference being experiment config changes)
  • Soon to be used to compare error rates with Control (only difference being source code changes)

This PR adds the rc-channel experiment to the page served at https://cdn.ampproject.org/experiments.html

Fixes #17706

[rsimha]

/to @cramforce @mrjoro @erwinmombay @jridgewell @choumx

[cramforce]

LGTM

@ampproject/tsc FYI