Add Client fallback to local PoW option

[cycraig]

Description of change

Exposes the fallback_to_local_pow configuration option on the ClientBuilder (Config for Wasm bindings), which is enabled by default. This allows disabling the behaviour of performing proof-of-work locally when publishing and no nodes support remote PoW.

Aside: we should consider refactoring Config, either by

• renaming it ClientConfig for clarity, or
• renaming it ClientBuilder and adding a build method for consistency, or
• replacing it with a duck-typed interface similar to other places in the Wasm bindings (may not be possible due to options with multiple parameters).

Added

• Add ClientBuilder::fallback_to_local_pow.
• Add Wasm Config.setFallbackToLocalPoW.

Links to any relevant issues

Fixes #667.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Wasm tests and examples pass locally.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes