[socketmode] Add methods with passing context

[im-kulikov]

There's no chance to gracefully shutdown socketmode.Client. (bug?)

• Add methods with passing context

  • added socketmode.Client.RunContext
  • added socketmode.Client.OpenContext
  • changed socketmode.Client.openAndDial to pass context
  • added test coverage for socketmode.Client.openAndDial
  • added test coverage for socketmode.Client.RunContext
  • added test coverage for socketmode.Client.OpenContext

• Use DialContext instead of Dial

• Graceful shutdown websocket.Conn (websocket connection freeze on conn.ReadJSON)

  • returned context errors to stop reconnect
  • closed channel of messages when exit from Client.run
  • closed websocket connection when context canceled

• mark test only for go 1.13 and higher

• fix typo in json tag

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Should this be an issue instead

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[im-kulikov]

test failed only for go1.12 (go1.13 is minimal according to go.mod)

[kanata2]

Thanks! (I'm planning to drop support of Go 1.12 in v0.9.0.)

[im-kulikov]

Is there any what I can do to be merged?

[kanata2]

LGTM :) Thanks!