fix(typings): export extraHeaders option

[justLuiz]

related: #1405

Note: the socket.io.js file is the generated output of make socket.io.js, and should not be manually modified.

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

extraHeaders is not exported in type definition.

New behavior

extraHeaders is now exported in type definition.

Other information (e.g. related issues)

[justLuiz]

this type, it should be part of the engine.io-client package but it is not a typescript yet, you may need to move these types later, taking care not to break current consumers

[darrachequesne]

Thanks!