Add builder pattern for Instance

[kozabrada123]

Adds a builder pattern for Instance - InstanceBuilder - which can be used to more precisely specify options before creating the Instance.

Adds the options to:

• disable automatically detecting the software with with_software,
• specify gateway options with with_gateway_options,
• add default event observers with .default_gateway_events,
• set default telemetry data (and telemetry sent in instance requests) with with_client_properties and
• skip optional requests which may reveal we are using chorus (see Ensure maximum Discord-API mimickry #584) with skip_fetching_ratelimits, skip_fetching_general_info & skip_optional_requests

Replaces Instance::from_url_bundle with InstanceBuilder::build as the main Instance initialization method

Changes Instance::from_url_bundle and Instance::new to no longer have GatewayOptions as their 2nd argument (now you can specify that in InstanceBuilder, and that argument really had no place there.)

To update, change:

• Instance::new("example.com", None) to Instance::new("example.com") or to InstanceBuilder::new("example.com".to_string()).build()
• Instance::new("example.com", Some(options)) to InstanceBuilder::new("example.com".to_string()).with_gateway_options(options).build()
• Instance::from_url_bundle(urls, None) to Instance::from_url_bundle(urls) or to InstanceBuilder::from_url_bundle(urls).build()
• Instance::from_url_bundle(urls, Some(options)) to InstanceBuilder::from_url_bundle(urls).with_gateway_options(options).build()