Cluster seed validation for CLI args (and interactive)

[StuartHarris]

This PR implements some more of #192 by introducing ClusterSeed for the wash call command args and interactive input.

Note there is a remaining question about what a default ClusterSeed should be (if anything), as this was previously an empty string (which won't parse now).

• introduce ClusterSeed
• write some unit tests for Id and Seed parsing
• revisit ClusterSeed::default() (wraps String::default())

Also added a .rustfmt.toml to allowing merging imports, but happy to revert this if you think the imports are now "too merged" :-)

[StuartHarris]

@brooksmtownsend opening this up for review, but would love to get your opinion on what ClusterSeed::default() should be. Before, it was an empty string (although not in a happy code path), so it may not matter what it is in reality (but I had to make it something that would now parse, so I made is SC + 54 x 0 :-)

[autodidaddict]

LGTM, though I'd like @brooksmtownsend to double-check this to make sure it works fine in conjunction with the automatic configuration reading ~/.wash/host_config.json

[brooksmtownsend]

As far as I can tell this looks good with wash ctx. Turns out seed keys are 58 characters so valid keys won't parse with this iteration. After doing some testing I found a few more requests.