add frozen abi requirements

[buffalojoec]

Problem

The Vote program uses solana-frozen-abi to define ABIs for a few different
types. In order to use it, we need to include the build script and the
specification annotation.

Summary of Changes

Add the build script and the specification annotation, setting up the crate for
solana-frozen-abi.

[buffalojoec]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• process update commission #10
• process update validator identity #9
• process authorize #8
• process initialize account #7
• init processor #5
• state: add vote_state #11
• init instructions #4
• init new state layout #3
• init errors #2
• add frozen abi requirements #1 👈 (View in Graphite)
• temp: pin versions to 2.0 from repo #6
• main

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