Call FlagStringer in String() method of slice flags

[fjl]

What type of PR is this?

• bug/cleanup

What this PR does / why we need it:

The default help template relies on the String() method of Flag to render the flag. For most flag types, String() indirects through FlagStringer, so that is the best place to customize flag rendering.

FlagStringer was not called for slice flags because their help output differs from other flags in two ways: there can be multiple default values, and the flag name is shown two times to indicate that the flag can be specified multiple times.

To make multiple values work in the FlagStringer, I simply changed GetValue() to return all values. This should be OK since the purpose of GetValue() is rendering documentation.

Showing the flag more than once is achieved through a new interface, DocGenerationSliceFlag, which the FlagStringer uses to decide whether the flag is a slice flag type. I'm not entirely happy with adding a new interface, but
it seems like the only fully backwards-compatible solution.

[fjl]

Some more context about this PR. We use a custom FlagStringer function in go-ethereum, and its output looks like this:

...   
   API AND CONSOLE
   
    --authrpc.addr value           (default: "localhost")
          Listening address for authenticated APIs
   
    --authrpc.jwtsecret value     
          Path to a JWT secret to use for authenticated RPC endpoints
...

When we added a StringSliceFlag (--header), I noticed that it breaks our help output:

...
    --port value                   (default: 30303)
          Network listening port
   
    --v5disc                       (default: false)
          Enables the experimental RLPx V5 (Topic Discovery) mechanism
   --header value  Pass custom headers to the RPC server wheng using --remotedb or the geth attach console.  (accepts multiple inputs)
   
   PERFORMANCE TUNING
   
    --cache value                  (default: 1024)
          Megabytes of memory allocated to internal caching (default = 4096 mainnet full
          node, 128 light mode)
...

This PR provides me with a way to fix the issue on our side. With the changes applied, our output looks aligned by default, and I can add custom formatting for slice flags.

[fjl]

If the new interface type is not wanted, I'm also happy to port this patch to the v3 branch and extend the DocGenerationFlag interface there.

[dearchap]

@fjl Thank you so much for this.