Allow setting websocket connection parameters

[HaraldNordgren]

We are having an issue with authorization when using subscriptions in genqlient.

Our mobile app team is using Apollo for GraphQL requests, and Apollo seems to sends its auth in a way that is different from how genqlient does it:

https://www.apollographql.com/docs/react/data/subscriptions#5-authenticate-over-websocket-optional

This is causing a problems, because we use genqlient for testing the backend.

It seems that the way Apollo is implemented, authentication (and other headers) are not actually sent as HTTP headers, but as connection parameters instead.

[HaraldNordgren]

Ping @benjaminjkraft and @matthieu4294967296moineau

[HaraldNordgren]

Since there was never a a release that includes NewClientUsingWebSocket, we should be free to update the signature if we want, and possible only have one function:

func NewClientUsingWebSocket(endpoint string, wsDialer Dialer, headers http.Header, connParams map[string]any) WebSocketClient

[HaraldNordgren]

A big gotcha regarding HTTP headers and connection parameters, that I realized recently, is that websocket requests in 99desgins gqlgen enter a different path, going through here:

	gqlgenServer.AddTransport(transport.Websocket{
		InitFunc: func(ctx context.Context, initPayload transport.InitPayload) (context.Context, *transport.InitPayload, error) {
			// initPayload contains connection parameters
		},
	})

than the middleware path that would normally be expected:

	r.With(ah.VerifyAuthMiddleware).Method(http.MethodPost, "/v1", c.GraphServer)
	r.With(ah.VerifyAuthMiddleware).Method(http.MethodGet, "/v1", c.GraphServer)