feat(connector): support OAuth2 scope preservation and incremental auth

Extend the OAuth2 connector so a reconnect can request the union of
previously granted and newly requested scopes without losing either.
Four related changes:

- Connection gains Scopes() []string so callers no longer need a type
  switch to reach the scope set. OAuth2Connection and APIKeyConnection
  implement it; SlackConnection inherits via embedding.
- OAuth2State carries RequestedScopes and CompleteWithState falls
  back to it when the provider omits the scope field (RFC 6749 §5.1
  allows this when granted equals requested). Without the fallback
  the stored Scope would be empty and the next reconnect would have
  no diff base.
- providerDefinition gains SupportsIncrementalAuth, set only for
  Google Workspace. When the flag is true and the caller passes
  InitiateOptions.IncludeGrantedScopes, the auth URL carries
  include_granted_scopes=true and the prompt=consent param is
  dropped so reuse flows see only the delta consent screen.
- InitiateOptions gains ConnectorID so the reconnect case is passed
  explicitly instead of relying on the caller to mutate r.URL.Query.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/connector/providers.go

@@ -26,10 +26,11 @@ type (
 	// initiate time via InitiateOptions, since the same provider may be used
 	// in multiple contexts requiring different scope sets.
 	providerDefinition struct {
-		AuthURL           string
-		TokenURL          string
-		ExtraAuthParams   map[string]string
-		TokenEndpointAuth string // "post-form" (default), "basic-form", or "basic-json"
+		AuthURL                 string
+		TokenURL                string
+		ExtraAuthParams         map[string]string
+		TokenEndpointAuth       string // "post-form" (default), "basic-form", or "basic-json"
+		SupportsIncrementalAuth bool
 	}
 )
 
@@ -79,6 +80,7 @@ var (
 				"access_type": "offline",
 				"prompt":      "consent",
 			},
+			SupportsIncrementalAuth: true,
 		},
 		"LINEAR": {
 			AuthURL:  "https://linear.app/oauth/authorize",
@@ -98,5 +100,6 @@ func ApplyProviderDefaults(provider string, redirectURI string, c *OAuth2Connect
 		c.TokenURL = def.TokenURL
 		c.ExtraAuthParams = def.ExtraAuthParams
 		c.TokenEndpointAuth = def.TokenEndpointAuth
+		c.SupportsIncrementalAuth = def.SupportsIncrementalAuth
 	}
 }