supabase / auth / 28583843677

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feat(linking): add experimental provider linking-domain groups (#2604)

## What kind of change does this PR introduce?

Feature

## What

Adds an experimental provider **linking-domain** map so independent
providers can be grouped to share account linking. Providers in the same
domain link to one another (matching verified email → same account) but
stay isolated from the default email-linked pool and from SSO.

```
GOTRUE_EXPERIMENTAL_PROVIDER_LINKING_DOMAINS="provider_a=social,provider_b=social"
```
## Why
The previous `GOTRUE_EXPERIMENTAL_PROVIDERS_WITH_OWN_LINKING_DOMAIN`
only let a provider own an isolated domain named after itself, two
distinct providers couldn't share one. This generalizes it to an
explicit provider→domain map.

## Notes
- Backward compatible: the old list is deprecated and auto-migrated in
`ApplyDefaults` to `{provider: provider}`; explicit map entries win. No
behavior change for existing deployments. (old list handling will be
removed in a later pr)
- SSO and `"default"` resolution are unchanged.

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