Add dynamic completions for target names in `rustup target` sub-command

[FranciscoTGouveia]

Currently, rustup target add does not support completions for target names.

We should use clap's dynamic completions for this, possibly extending them to other subcommands (see #2268).

[Cloud0310]

I am interested with this as well, maybe we can do this together? @FranciscoTGouveia

[rami3l]

@Cloud0310 I think it'd be cool if you can attack #2268 and @FranciscoTGouveia on rustup target *. There will inevitably be an overlap somewhere but I believe one PR can be easily rebased on top of the other. How does that sound?

[Cloud0310]

Sounds good to me, I will try to fix the part that I am interested: dynamic completion is not been enabled in rustup, causing command like rustup component <Tab>'s completion is wrong.

[rami3l]

@FranciscoTGouveia Just for the heads up: we have added the basics of dynamic completion in #5033.

[Cloud0310]

Here's more context, you should see the limitation from clap.
I suggest once @FranciscoTGouveia is done with issue, we should add docs for this problem.