Implement a mechanism so multiselect option can have all available choices

[hunghvu]

Related to #112.

[hunghvu]

Sample of idea.

const payload = require('payload');

payload.collections['collection_name'].Model.distinct('field_name', {status: {$eq: '*condition*'}}, async (err, data) => {
  if (err) {
    // handle error
  } else {
    // work with unique data values
    const uniqueValues = [...new Set(data)];
    console.log(uniqueValues);
  }
});

[hunghvu]

Commit 6a1992e reduces payload from the CMS from 80kb to 20kb, and cuts a huge amount of DB operations too.

[hunghvu]

Consider as done.