Expose join capabilities api

[rafaeljesus]

• Expose more fields for generating proper join between relationships
• Add docs

The current implementation does not allow filtering complex relationships, this PR add capabilities for the klepto users to proper filter table with relationships. Let's take this scenario:

I want to dump only the active users with their orders and it's order_items

And need to anonymise the email and firstName fields respectively

Given the klepto config

[[Tables]]
  Name = "users"
  [Tables.Anonymise]
    email = "EmailAddress"
    firstName = "FirstName"
  [Tables.Filter]
    Match = "users.active = TRUE"

[[Tables]]
  Name = "orders"
  PrimaryKey = "id"
  [[Tables.Relationships]]
    Table = "orders"
    ForeignKey = "user_id"
    ReferencedTable = "users"
    ReferencedKey = "id"
  [Tables.Filter]
    Match = "users.active = TRUE"

[[Tables]]
  Name = "order_items"
  PrimaryKey = "id"
  [[Tables.Relationships]]
    Table = "order_items"
    ForeignKey = "order_id"
    ReferencedTable = "orders"
    ReferencedKey = "id"
  [[Tables.Relationships]]
    Table = "orders"
    ForeignKey = "user_id"
    ReferencedTable = "users"
    ReferencedKey = "id"
  [Tables.Filter]
    Match = "users.active = TRUE"

Tables.Relationships fields are necessary to generate joins between tables behind the scenes e.g

generated query for dumping users:

select * from users where users.active = true

generated query for dumping orders from the users:

select * from orders
join users where users.id = orders.user_id
where users.active = true

generated query for dumping order_items from the user orders:

select * from order_items
join orders where orders.id = order_items.order_id
join users where users.id = orders.user_id
where users.active = true

FYI dumping large databases specially with the mysql-drive is pretty painful, a lot of bad conn errors are raised after some time dumping, this is where the filter comes in

[codecov]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   32.39%   32.39%           
=======================================
  Files           5        5           
  Lines         213      213           
=======================================
  Hits           69       69           
  Misses        140      140           
  Partials        4        4

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