cli: add feature revoke

[buffalojoec]

Problem

The new Feature Gate Core BPF program is live at Feature111111... on both testnet and devnet, and will soon be on mainnet-beta.

With this new program, we can revoke features that are pending activation. The program has only one instruction, RevokePendingActivation, which simply "closes" a newly created feature account by:

• Deallocating its data
• Burning its lamports
• Reassigning it to System

However, core contributors who wish to use this new capability have no way of doing so from the CLI.

Summary of Changes

Add a new command to the solana feature CLI that invokes the RevokePendingActivation instruction on the Feature Gate program to revoke a pending activation.

For more information about this instruction, see the program's instruction source. However, in summary, once a feature is activated with solana feature activate, it can be revoked at any time before the epoch rollover, while the state is still zeroed. Once the epoch concludes and the feature is activated by the runtime, the account is bestowed with valid Feature state and thus can no longer be revoked.