chore: refactor codersdk to use SessionTokenProvider (#19565)

Refactors `codersdk.Client`'s use of session tokens to use a `SessionTokenProvider`, which abstracts the obtaining and storing of the session token.

The main motiviation is to unify Agent authentication an an upstack PR, which can use cloud instance identity via token exchange, rather than a fixed session token.

However, the abstraction could also allow functionality like obtaining the session token from other external sources like the OS credential manager, or an external secret/key management system like Vault.

codersdk/workspacesdk/workspacesdk.go

@@ -215,12 +215,12 @@ func (c *Client) DialAgent(dialCtx context.Context, agentID uuid.UUID, options *
 		options.BlockEndpoints = true
 	}
 
-	headers := make(http.Header)
-	tokenHeader := codersdk.SessionTokenHeader
-	if c.client.SessionTokenHeader != "" {
-		tokenHeader = c.client.SessionTokenHeader
+	wsOptions := &websocket.DialOptions{
+		HTTPClient: c.client.HTTPClient,
+		// Need to disable compression to avoid a data-race.
+		CompressionMode: websocket.CompressionDisabled,
 	}
-	headers.Set(tokenHeader, c.client.SessionToken())
+	c.client.SessionTokenProvider.SetDialOption(wsOptions)
 
 	// New context, separate from dialCtx. We don't want to cancel the
 	// connection if dialCtx is canceled.
@@ -236,12 +236,7 @@ func (c *Client) DialAgent(dialCtx context.Context, agentID uuid.UUID, options *
 		return nil, xerrors.Errorf("parse url: %w", err)
 	}
 
-	dialer := NewWebsocketDialer(options.Logger, coordinateURL, &websocket.DialOptions{
-		HTTPClient: c.client.HTTPClient,
-		HTTPHeader: headers,
-		// Need to disable compression to avoid a data-race.
-		CompressionMode: websocket.CompressionDisabled,
-	})
+	dialer := NewWebsocketDialer(options.Logger, coordinateURL, wsOptions)
 	clk := quartz.NewReal()
 	controller := tailnet.NewController(options.Logger, dialer)
 	controller.ResumeTokenCtrl = tailnet.NewBasicResumeTokenController(options.Logger, clk)