Generate abi_gen workspace package

Created by: ascjones

As a follow up to #33 we should investigate the possibility of generating the files in .ink/abi_gen.

• Generate files currently in .ink/abi_gen in template
  • Would require determining the contract type for main.rs: let abi = <contract::{{camel_name}} as ink_lang::GenerateAbi>::generate_abi(); which will not neessarily be the same name as the contract package.
• Add or amend workspace section with ink_abi as a member
• Check build caching works