Extrinsics: allow specifying contract artifact directly

[ascjones]

Extrinsic commands need to locate contract artifacts: for upload we require the contract Wasm and for instantiate and call the contract metadata. Currently this is designed to be similar to other cargo commands: providing a --manifest-path argument to point to the Cargo.toml of the contract project, if not present assuming the working directory is the project root. From there it can locate the artifacts in the ./target/ink/ directory. This is good for a development workflow from inside a project dir like:

cargo contract build --release
cargo contract upload --suri //Alice
cargo contract instantiate ...

Or from a different directory relative to the contract project directory:

cargo contract build --manifest-path ../my-contract/Cargo.toml --release
cargo contract upload --manifest-path ../my-contract/Cargo.toml --suri //Alice
cargo contract instantiate --manifest-path ../my-contract/Cargo.toml ...

However, this workflow is designed for a developer who is working with the contract source locally.

This PR adds the ability to provide the path to a contract artifact file directly, for use cases where a user does not have the contract source project locally.

For all extrinsics, there is now an optional positional argument for the path to an artifact file. This can be one of .wasm (code only), .json (metadata only) or .contract (metadata + wasm). e.g.

cargo upload ../path/to/mycontract.wasm
cargo instantiate ../path/to/mycontract.contract
cargo call ..path/to/metadata.json

When specified, it will use that artifact file. When not specified it will fall back to the existing --manifest-path method.

Note: The existing instantiate command has a --wasm-path argument. This has been replaced by this new positional argument for consistency across the extrinsic commands.

Closes #892