drep id: have --output-hex and --output-bech32 instead of --output-format [hex|bech32]

[smelc]

Context

While writing #1009, I noticed that drep id has the following interface:

                                               [--output-format STRING]
...
  --output-format STRING   Optional drep id output format. Accepted output
                           formats are "hex" and "bech32" (default is "bech32").

Whereas we've decided (see all PRs implementing #566) to avoid STRING arguments for format flags, i.e. in this case we rather want to have --output-hex and --ouput-bech32.

Change planned

Change drep id to have flags --output-hex and --output-bech32 instead of --output-fomat STRING.

[smelc]

I will do this one after #1009 is merged