New stellar-config crate

[willemneal]

What problem does your feature solve?

Initially the CLI was the only crate that needed to know about the config. But it should rather be a dependency so that other crates can read from a configuration to know about:

• Networks
• Keys
• Contract Aliases

One initial use case: Compile time info:

For example, the native SAC has a deterministic Address given a network. So a contract compiled for testnet should be able to statically include it. However, the contract needs to know what network it's being compiled for, which could be provided through stellar-config.

• Improve errors: See [Bug] --network with a non-existent network should return better error #1297

What would you like to see?

Standardize set of configurations to be collected into one crate.

What alternatives are there?