Support creating catalogs from JSON objects

[ppaglilla]

Adds support for creating catalog that use resources defined in a JSON object. This is primarily intended for testing purposes, so that users don't need to write their own [BigQuery/Spanner]ResourceProvider for simple testing.

It achieves it by:

• Introducing the CatalogResources dataclass and a way to deserialize from a JSON object
• Creates resource provider implementations for both BigQuery and Spanner that use a CatalogResources object

Example:

String json = "{\n"
    + "  \"tables\": ["
    + "    {"
    + "      \"name\": \"projectId.dataset.table\","
    + "      \"columns\": ["
    + "        { \"name\": \"column\", \"type\": \"INT64\" },"
    + "        { \"name\": \"column2\", \"type\": \"TIMESTAMP\", \"isPseudoColumn\": true }"
    + "      ]"
    + "    }"
    + "  ]"
    + "}";
CatalogResources resources = CatalogResources.fromJson(json);
BigQueryCatalog catalog = BigQueryCatalog.usingResources("projectId", resources);