Create MultiTableMetadata class

[amontanez24]

Problem Description

As a user, it would be useful to have a programmatic way to interact with the metadata for all of the tables in my dataset.

Expected behavior

• In the metadata module, add a class called MultiTableMetadata
• The class needs to store a dictionary that maps all of the table names to their SingleTableMetadata class instances and another dictionary that stores the relationships.
• The __init__ requires no parameters
• There should be a to_dict method that returns the underlying dictionaries in the proper json format

>>> python_dict = metadata.to_dict()
{
  "tables": {
    "accounts": { <Table Spec> },
    "branches": { <Table Spec> }
  }, 
  "relationships": [
    { <Relationship Spec>},
    { <Relationship Spec> },
  ],
}

• There should be an internal _load_from_dict(dict) method that returns a class instance with the values set based on a provided python dictionary that matches the json spec. The instance should call the SingleTableMetadata._load_from_dict to create the SingleTableMetadata instances.

metadata = SingleTableMetadata._load_from_dict(dict)

• The __repr__ method should be overridden to print the full dictionary representation of the json for the MultiTableMetadata.

>>> print(metadata)
{
  "tables": {
    "accounts": { <Table Spec> },
    "branches": { <Table Spec> }
  }, 
  "relationships": [
    { <Relationship Spec>},
    { <Relationship Spec> },
  ],
}