cli: Write output to file

[lexnv]

This PR adds the ability to output the result of the subxt/cli metadata command to a dedicated file.

This is in response to #969, where the output from the Windows process to the file results in metadata byte differences.

With this patch applied:

$> Get-FileHash .\win.scale

Algorithm       Hash                                                                   Path
---------       ----                                                                   ----
SHA256          637D22238919A0293847BB0DA3F4E3E3FCF0711AF07DE7BF3E8527437962E8BE       D:\subxt\cli\win.scale


$> shasum -a 256 mac.scale                                                     
637d22238919a0293847bb0da3f4e3e3fcf0711af07de7bf3e8527437962e8be  mac.scale

Closes #969

[jsdw]

Lovely!