rev shelf_web_socket to 3.0 in prep for publishing

[devoncarew]

• rev shelf_web_socket to 3.0 in prep for publishing
• fix add type information to the webSocketHandler onConnection callback #457

Our various mono-repos (the sdk, google3, and flutter) have been preped for this change.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:shelf_web_socket	3.0.0	ready to publish	shelf_web_socket-v3.0.0
package:shelf	1.4.3-wip	WIP (no publish necessary)
package:shelf_packages_handler	3.0.3-wip	WIP (no publish necessary)
package:shelf_proxy	1.0.5-wip	WIP (no publish necessary)
package:shelf_router	1.1.5-wip	WIP (no publish necessary)
package:shelf_router_generator	1.1.1	already published at pub.dev
package:shelf_static	1.1.3	already published at pub.dev
package:shelf_test_handler	2.0.3-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.