Enable local proof-of-work fallback

[cycraig]

Description of change

This enables the pow-fallback feature for iota-client, which means publishing messages falls back to performing proof-of-work locally (on the client) when no nodes support remote proof-of-work.

NOTE: this changes the behaviour of the local_pow setting on the ClientBuilder, making it a preference whether or not to try remote PoW first before using local PoW rather than a hard setting. Since the pow-fallback is a compile-time feature we cannot change whether or not to fallback at runtime. This change may be temporary.

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

Tested with a local one-click-Tangle with remote PoW enabled and disabled on the Hornet node.

1. When remote PoW on the node is disabled and local PoW on the client is false without local fallback from this PR, we get the following error from the Client when trying to publish a new identity:

   Err > Error [ClientError]: Node error: Response error with status code 400: {"error":{"code":"400","message":"invalid parameter, error: proof of work is not enabled on this node: code=400, message=invalid parameter"}}

2. When remote PoW on the node is disabled with the local fallback enabled by this PR, the client performs PoW locally and successfully publishes the new identity (regardless of the local_pow variable).

3. When remote PoW on the node is enabled the client respects the local_pow variable as per the current behaviour.

Change checklist

Add an x to the boxes that are relevant to your changes.

• 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