feat(default-schema): allow addition of default schema through config

[rohhamh]

I faced a situation in which I had to change all queries to target a specific schema and I found no way to do so without changing the huge amount of knex query codes or editting the library method itself by adding this:

const Knex = require('knex')({
  client,
  connection,
})

const QueryBuilder = require('knex/lib/query/builder')
Knex.client.constructor.prototype.queryBuilder = () =>
  (new QueryBuilder(Knex.client)).withSchema('mySchema')

However, I think allowing the schema to be set from the very inital configuration like below can be of help to a few others like me.

const Knex = require('knex')({
  client,
  connection,
  schema: 'mySchema'
})

[siakc]

I had to use this patch in order to run some migrations on an Oracledb schema which had access to another schema. Please consider merging this.