Improve fixture generator tool

[afterdusk]

See #195 for context.

• Fixture reader so that users do not need to write their own parser. (Add fixture reader #200 )
• Support using a custom VRF key rather than the hardcoded one.
• Support using a custom hasher rather than Blake3.
• Better CLI binary name - it currently adopts the crate name (akd_integration_tests).
• Error handling instead of unwraps.
• Add mode to allow specification of a number of users to randomly generate updates for, without needing to list out the epochs or usernames.
• Document how crate should be versioned (see this comment)