feat: Add basic auth support

[cdvv7788]

Minor PR that adds support for basic auth and add a small example.

[markphelps]

Thanks for the PR @cdvv7788 !

Since Flipt Server doesnt natively support basic auth, is this for the case where you are running Flipt behind a reverse proxy such as nginx or caddy?

[cdvv7788]

Yep, running it behind nginx. May be a common enough case, and it is simple to support it :)

[markphelps]

@cdvv7788 what if instead we add the ability to set arbitrary headers? Adding support for manipulating headers in all clients seems more flexible / reasonable for us to maintain.

Especially since Flipt itself doesn't support basic auth, it might be a bit confusing to users of the SDKs if they see a BasicAuthentication method in the SDKs

[cdvv7788]

I don't mind either way. This is a thin wrapper on httpx, so other stuff will have to be implemented on the user side anyway. I created the PR because I already had done this, but feel free to close if it is not a good fit for the project 👍

[markphelps]

@cdvv7788 wdyt about changing this PR to allow setting the headers directly instead?

Then we can open up issues to do it in the other languages as well