Add example: Login with API token (CSR only)

[flosse]

Here is an example that demonstrates how you can use leptos for SPAs that use some kind of a API token (e.g. JWT).

Motivation:
I am considering porting an existing web application whose server/API cannot be changed and SSR is not needed initially.
To get a sense of what an example looks like beyond a counter example, I chose this scenario.
Since I'm new to FRP, I'm interested in any feedback that could make the code cleaner.

[gbj]

Hi! Could you maybe provide a little more commentary on how close you think this is to ready? It's quite a large example... I'm happy to sit down and really try to understand it but I also see various FIXME comments and things. Are you still working on it or do you want a review at this point?

[flosse]

Actually, it is already working but I'd not call it finished because of the three FIXME points for which I have not yet found a solution. Two of them could be phrased as questions:

1. How to read the initial value of an input field after the browser made an autofill?
2. How to call use_navigate outside of a Router?

The third is not really a problem but only something that feels a bit fragile.

So my suggestion is that I mark this PR as draft and I will get back to you as soon as I have solved the three issues so that I don't waste your time unnecessarily.

[flosse]

@gbj I think it's ready for review now. I expect some parts are not written in a leptops/FRP ideomatic way, but that is due to my lack of knowledge about it. So please be picky and point out anything to me that isn't pretty.

[gbj]

Honestly this looks pretty good to me. Thanks so much for the contribution!