[codegen] Make additionalSchemas configuration easier

[d4rekanguok]

Current minimal setup:

exports.default = {
  plugins: [{
    resolve: `gatsby-plugin-graphql-codegen`,
    options: {
      // (1)
      additionalSchemas: [{
        // (2)
        key: 'pokemon',
        // (3)
        fileName: './graphql-pokemon.ts',
        schema: 'https://graphql-pokemon.now.sh/',
        // (4)
        pluckConfig: {  //
          // config to ensure only queries using the `gql` tag are used for this schema
          globalGqlIdentifierName: 'gql',
          modules: [
            {
              name: 'graphql-tag',
              identifier: 'gql',
            },
          ],
        },
      }],
    }
  }]
}

Considering that most users would have only 1 additional schema, we should:

1. Check if additionalSchemas is an array or object, so user can just pass in an object directly
2. Remove key. Make key optional. Create a key automatically based on schema path
3. Make fileName optional. Use a sensible default like graphql-types-addition, if additionalSchemas is an array, append a number
4. Make pluckConfig optional. User only needs to pass in name & identifier

So ideally, most users should only need to do this:

exports.default = {
  plugins: [{
    resolve: `gatsby-plugin-graphql-codegen`,
    options: {
      additionalSchemas: {
        schema: 'https://graphql-pokemon.now.sh/',
        //  import gql from 'graphql-tag'
        gqlIdentifier: ['gql', 'graphql-tag']
      },
    }
  }]
}