sdk: Add macro to get Cargo.toml package metadata

[joncinque]

Problem

As part of the new format for solana programs, we want to use the [package.metadata] section in Cargo.toml to store Solana-specific information, such as the program id, as in https://github.com/solana-program/config/blob/396c398b558abb0b57c9c03e11f766417ce1fc94/program/Cargo.toml#L11-L12

Unfortunately, we need to specify this in two places since it isn't possible to just get the data from the Cargo.toml's metadata directly, ie https://github.com/solana-program/config/blob/396c398b558abb0b57c9c03e11f766417ce1fc94/program/src/lib.rs#L11

Summary of Changes

Following the implementation at https://gitlab.com/shanepearlman/cargo_meta, add a macro that extracts the data from Cargo.toml directly.

This is the first step, next we need to add a macro to generate a const pubkey that we can use with declare_id, ie. something like:

macro_rules! declare_pubkey {
    ($pk:expr) => {{
        let id_array = bs58::decode($pk.as_bytes())
            .with_alphabet(bs58::Alphabet::RIPPLE)
            .into_array_const_unwrap::<PUBKEY_BYTES>();
        Pubkey::new_from_array(id_array)
    }};
}

declare_id!(declare_pubkey!(package_metadata!("solana.program-id")));

cc @lorisleiva

Fixes #

[buffalojoec]

Neat! Looks good to me, just the CI fix.

[joncinque]

Should be ready for another look!