BaseHTTPClient: better constructor API, Default

[mvidner]

to enable

• connecting to alternative servers, either production or test mocks
• unauthenticated calls, either for the initial auth or test mocks

This is needed for both #1438 and #1495

In the end, the API changes only by adding a Default trait implementation which does no authentication.
Connecting to a mock server is achieved by assigning to the public base_url field.

[imobachgs]

Why is this field public? IMHO, BaseHTTPClient should offer the API that fulfills our needs, and the reqwest::Client should be just an implementation detail. If we want to replace reqwest with something else (e.g., ureq) we can do it easily (or if reqwest API changes it might be easy to adapt).

[mvidner]

It enables me constructing a mock client like this:

        let mock_client =  BaseHTTPClient {
            base_url: mock_url,
            ..Default::default()
        };

I chose this way because I did not want to come up with a special name for it.

Alternatively we can make it private again, remove the Default trait and make a set of specialized constructors, like new_unauthenticated_with_url here?

[mvidner]

Anyway, I consider BaseHTTPClient pretty volatile, subject to quick rewrite as its users need it

[mchf]

I was playing with an idea to implement some traits here and than do necessary changes in behavior in uderlying structs (which can even cover differences in using authorization token or not)

[mchf]

In my POV default (or bare in my PR) are the best names ...
... I was thinking about something like new -> authorized and default / bare -> nonauthorized or something more fancy with similar meaning. Simply because behavior of new is not that clear (that it expects authorization token somewhere already)

[imobachgs]

LGTM. As @mvidner said, this BaseHTTPClient is pretty volatile and we need to experiment a bit more. But by now, it looks good enough.