Allow to define summary for flag options.

[cristiand391]

Is your feature request related to a problem? Please describe.
If you have a command with a flag that accepts a set of options like this:
https://github.com/salesforcecli/plugin-deploy-retrieve/blob/cf94d9b10d1256e9cad96f75df3452094b9d4865/src/commands/project/deploy/start.ts#L121

you get those in zsh/powershell completion but plugin-autocomplete can't show any summary/description about them:

Describe the solution you'd like
Allow to define a summary for each flag option so that plugin-autocomplete can set it in completion functions (natively supported):

Describe alternatives you've considered
make flag.options an object containing one 1 object per option

'test-level': testLevelFlag({
      description: messages.getMessage('flags.test-level.description'),
      summary: messages.getMessage('flags.test-level.summary'),
      options: {
        TestLevel.NoTestRun: {
          summary: 'No tests are run'
        }
      },
      helpGroup: testFlags,
    }),

Additional context
We could also update the help logic so that it renders these summaries, in sf we just add all of them in the flag summary:
https://github.com/salesforcecli/plugin-deploy-retrieve/blob/cf94d9b10d1256e9cad96f75df3452094b9d4865/messages/deploy.metadata.md#flagstest-leveldescription