Support local configuration, not only global

[sandstrom]

The plan mentions supporting both global and local state.

braintrust-sdk-ruby/.plan/.PLAN.md

Lines 16 to 28 in 1929a20

	```ruby
	# Simple case - sets global state (default)
	Braintrust.init(
	api_key: ENV['BRAINTRUST_API_KEY'],
	project: "my-project"
	)
	# Explicit state (for testing, multi-tenant)
	state = Braintrust.init(
	api_key: "key",
	project: "project",
	set_global: false
	)

This is a very good pattern.

But currently, only global seems to be supported. At least that's all that's in the readme.

Also, if you support local state, I would suggest the traditional class pattern with new (idiomatic ruby):

# Simple case - sets global state (default)
Braintrust.init(
  api_key: ENV['BRAINTRUST_API_KEY'],
  project: "my-project"
)

# Explicit state (for testing, multi-tenant)
braintrust_instance = Braintrust.new(
  api_key: "key",
  project: "project",
  set_global: false
)

(the one in the plan is even broken, it wouldn't work at all as mentioned in the plan)

[sandstrom]

ping Alan R. Fachini (@alfakini) Maybe this is relevant for your problem too? I saw you mentioned multiple projects in a single ruby process.