Create ledger and keystore cargo features

[ifropc]

What problem does your feature solve?

We have added support for native keystore in 22.3 (and previously for Ledger), however both of this features require extra native libraries to compile. With introduction of keystore, we saw a lot of pipelines breaking, and that also affects downstream users. (see #1910 )
However, only fraction of users will use keystore or ledger (and only those users should install optional dependencies)
We will still continue to produce full-featured binaries, but for people who are installing using cargo those features should be opt-in.

What would you like to see?

Create a cargo --feature sets for both Ledger and Keystore

What alternatives are there?

Install required lib dependencies before using cargo install

[leighmcculloch]

I think we could be encouraging use of the stellar/stellar-cli action that automatically installs the binary, rather than introducing more complexity into the main build.

At some point we introduced the opt feature for this same reason, so we could do same, but that was before we offered reliable binary builds and easy installs of them.

But I think it's fine to put more things behind a feature, maybe make the feature 'extra-keystores' or something like that, that can group all the extra keystores, rather than separate features for each.

[ifropc]

I think most people are not aware of the action, and it's not very discoverable (we might want to update our README). I'd imagine most people would just be using cargo install
I agree that just one feature flag is better than multiple in this case and makes it more simple to work with

[leighmcculloch]

Maybe we can ditch the opt feature and have a sys or additional-libs feature for any feature requiring libs.

[leighmcculloch]

Discoverability is a good point. We should add developer docs about using the CLI in GitHub Actions. And update the readme as you say.

[ifropc]

Updated docs in #1932 and stellar/stellar-docs#1389

[github-actions]

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