Validate checksum via `bundle-checksum lint`

What does this MR do and why?

This MR enhances the command bundler-checksum lint to also verify gem's checksum which is useful when upgrading gems.

Resolves #419326 (closed).

How to set up and validate locally

# Change checksum of `unf` in `Gemfile.checksum`

$ bundle exec bundler-checksum lint
ERROR: Invalid checksum for gem `unf` (0.1.4 ruby)

Expected: 4999517a531f2a955750f8831941891f6158498ec9b6cb1c81ce89388e63022X
  Actual: 4999517a531f2a955750f8831941891f6158498ec9b6cb1c81ce89388e63022e

Please run `bundle exec bundler-checksum init` to add correct checksums.

# Remove platform-specific entry for `unf` from `Gemfile.checksum`

$ bundle exec bundler-checksum lint
ERROR: Missing checksum for gem `unf` (0.1.4 ruby)

Please run `bundle exec bundler-checksum init` to add correct checksums.

# Downgrade default gem `ipaddr` to `1.2.1` in `Gemfile` and run `bundle`

$ bundle exec bundler-checksum lint
ERROR: Missing checksum for gem `ipaddr` (1.2.1 ruby)

Please run `bundle exec bundler-checksum init` to add correct checksums.

MR acceptance checklist

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