IVY-1280 Support preemptive authentication

[twogee]

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I adopted a patch by @AurelienLourot

[lourot]

Note that I didn't push it upstream because of this open point: https://github.com/tt-gf/ant-ivy/pull/2#discussion_r135625987

So this might break some authentication schemes, I'm not really sure.

EDIT: we have been using it at TomTom for a while, but we use only the Basic auth scheme.

[twogee]

I have noticed that. Perhaps it's enough to document that preemptive authentication only works with basic scheme.

[twogee]

Looking at examples, digest authentication uses the same cache, only sets a different scheme.

[twogee]

I would like to add the preemptive authentication flag to the newly minted TimeoutConstrainedURLHandler to avoid BWC issues. Any objections, @jaikiran ?

[twogee]

@AurelienLourot we moved the discussion to JIRA