feat(avm): add avm self-update command and passive version-check warning

[tiago18c]

• avm self-update updates avm itself via cargo install --git; supports --pre-release (latest pre-release tag) and --bleeding-edge (master branch)
• On every command invocation, checks if a newer avm version is available and warns the user
• Update check uses a 3-second timeout and a 1-hour cache ($AVM_HOME/.update-check) to avoid blocking the CLI for too long and too often
• Errors during the check are persisted so retries are suppressed within the cache window, with a countdown shown to the user

[vercel]

@tiago18c is attempting to deploy a commit to the Solana Foundation Team on Vercel.

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