make cargo-near-buid::extended::build api ergonomic by using sane defaults

[dj8yfo]

A build with https://docs.rs/cargo-near-build/0.4.2/cargo_near_build/extended/index.html requires many additional
params to be specified.
Ideally, it should only require specifying manifest_path of target subcontract, that is all.
Probably each bullet can be resolved by a separate pr.

• automatically compute override_nep330_contract_path based on path of target contract relative to root of repo, if a repo is detected
• a good default for override_cargo_target_dir is present in chore: add skip of sub-build on abi generation to circumvent cargo-near#287 NEAR-DevHub/neardevhub-treasury-dashboard#212 (comment)
  • it can also probably be removed from public api by https://bon-rs.com/reference/builder/member/skip and only the default value always used as an irrelevant implementation detail
• a good default for stub_path is present in chore: add skip of sub-build on abi generation to circumvent cargo-near#287 NEAR-DevHub/neardevhub-treasury-dashboard#212 (comment)
• a good default for result_env_key might be "BUILD_RS_SUB_BUILD_{target_crate_name}_WASM"
• a default for build_skipped_when_env_is should be empty vec![] but current values suggested in doc should be suggested in doc too
• workdir (computed based on manifest_path) should be added by default to rerun_if_changed_list, and vector from any non-default value extended into final value (thus containing workdir + api-user supplied value)