Support webhooks in Data Connect behind an experiment flag.

[rosalyntan]

Description

This change adds initial webhooks support in Firebase Data Connect CLI behind an experiment flag.

Notable behavioral changes:

• If the experiment flag is enabled, firebase init --only dataconnect will create a dataconnect.yaml file with a schemas field, instead of schema (all changes are backwards compatible with the previous dataconnect.yaml file)
• If secondary schemas are present, the CLI will deploy them in addition to the main schema as part of firebase deploy --only dataconnect

Outstanding TODOs:

• Download and update dataconnect.yaml file with existing secondary schemas as part of firebase init
• Add a command to init secondary schemas
• Update dataconnect:services:list to list secondary schemas as well
• Supporting secondary schemas in MCP tooling

Scenarios Tested

Sample Commands

[fredzqm]

Solid work!

No blocking comments. A few nits. Feel free to defer though.