Add configurable http connection

[andig]

Fix #60

By passing e.g. an oauth2 client, the http connection connection can securely connect.

Example:

ctx := context.Background()
conn, err := signalr.NewHTTPConnection(ctx, "https://api.easee.cloud/hubs/chargers", signalr.HTTPClientOption(oauth2Client))
if err != nil {
	return c, err
}

client, err := signalr.NewClient(ctx, conn)
if err != nil {
	return c, err
}

I would also suggest to remove the NewHTTPClient as it has only limited flexibility (breaking change).

[philippseith]

Nice job! Could you please add a test for NewHTTPConnection?

[philippseith]

remove the NewHTTPClient:

As it is only used in tests, I don't bother :-)

[andig]

remove the NewHTTPClient:

As it is only used in tests, I don't bother :-)

Maybe I'm doing this wrong- how would you connect to the API above instead?

[philippseith]

No, no, you are doing perfectly right. I meant removing the existing NewHTTPClient doesn't bother me.
When writing this package, my main objective was the server. But while writing it, I realised that there should be a simple go client to test the server. And it turned out that server client share a lot of code and are nearly interchangeable.

[andig]

@philippseith do you want to start CI?

[andig]

Tests are updated, too and http client removed