package-metadata: Add macro to define program id from Cargo.toml

[joncinque]

Problem

This is a remake of #1483 but using a new crate instead of adding to solana_program.

We've added in the preliminary scaffolding in #1464 and #1465, but we still don't have the last bit to declare the program id in Cargo.toml.

Summary of Changes

Add the macro. As part of this, we have a few additions:

• Move the documentation for the macro and re-export in solana-program. This seems to be how we do all the other macros
• Add Pubkey::from_str_const
• Add a little extra helper macro to combine everything
• Changed one of the test programs to show the new way to declare the program id.

As noted in Pubkey::from_str_const, it's currently impossible to do a length check, which isn't great. We'll see if the upstream PR get accepted. I can keep this in draft until that goes through. I can also move that to a separate PR.

cc @lorisleiva

[buffalojoec]

Lgtm again, just need the upstream PR to get reviewed.