Add support for distributed apps

[lalabuy948]

• Added PostgresDB backend support issue 8
• Added example apps/integrations
• Added configurable cache
• Cache will be disabled by default issue 13

[!NOTE] In case you would like to enable it back you can optionally add this line to the config

config :phoenix_analytics,
  app_domain: "example.com",
  duckdb_path: "analytics.duckdb",
  cache_ttl: System.get_env("CACHE_TTL") || 120 # seconds # <- this one

[!IMPORTANT]
Breaking changes

• System.get_env("DUCK_PATH") renamed to System.get_env("DUCKDB_PATH")
• config :phoenix_analytics, database_path -> config :phoenix_analytics, duckdb_path
• cache disabled by default issue 13

[!NOTE]
The only thing you need to enable Postgres backend to add one line to config:

config :phoenix_analytics,
  app_domain: "example.com",
  duckdb_path: "analytics.duckdb",
  postgres_conn: "dbname=postgres user=phoenix password=analytics host=localhost" # <- this one

Here is full list of Postgres parameters.