Allow setting target directory for contract builds

[davidsemakula]

Summary

Related to use-ink/ink#2520

• [y] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependent on the specific version of ink or pallet-contracts?

Description

• Allow setting target directory for contract builds by introducing a target_dir field in ExecutionArgs
  • This is currently intended to allow e2e tests infra to share intermediate build artifacts across all contract dependency builds (see details), so it's currently not exposed as a CLI arg
• Share target directory for intermediate build artifacts for contracts in workspaces (similar to how cargo handles target directory in workspaces)
  • Introduces an artifact_directory field in CrateMetadata allowing only the final build artifacts (i.e. contract binary, metadata e.t.c) to be placed in a separate directory for each workspace member (also similar to cargo's currently unstable --artifact-dir , but only used internally)

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[AlexD10S]

LGTM!