Add a testing provider for downstream packages to consume

[atheriel]

Following up on conversation in r-lib/httr2#729: it's hard to write unit tests that confirm that telemetry is working right now.

Some of the existing otelsdk tests use a pattern of writing spans to disk and then parsing them back to confirm that spans are created correctly:

otelsdk/tests/testthat/test-tracer.R

Lines 12 to 14 in 3162000

	tmp <- tempfile(fileext = "otel")
	on.exit(unlink(tmp), add = TRUE)
	trc_prv <- tracer_provider_stdstream_new(tmp)

However, I think we should have an in-memory tracer provider specifically for testing in otel. This is a fairly common pattern in the ecosystem -- see e.g. Logfire, which has a nice API that we could emulate. I noticed the Go SDK has a testing provider as well.

[gaborcsardi]

#14 adds an in-memory tracer provider, but we still need improvements: #16.