allow LOGFIRE_BASE_URL to accept grpc endpoints

[davidhewitt]

Related to #1042

The idea (from offline discussion) is that we can set LOGFIRE_BASE_URL to a grpc endpoint and everything can "just work", with the caveat that we wouldn't be able to do any token checking.

This is sufficient for a local test script to work for me:

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "logfire[grpc]",
# ]
# ///

import os

import logfire

os.environ['LOGFIRE_BASE_URL'] = 'grpc://localhost:4317'

logfire.configure(
    # Setting a service name is good practice in general, but especially
    # important for Jaeger, otherwise spans will be labeled as 'unknown_service'
    service_name='my_logfire_service',
)

with logfire.span('This is a span'):
    logfire.info('Logfire logs are also actually just spans!')

---

Steps to complete before merge:

• Update documentation for OTEL collectors
• Testing - not sure how much we want this covered? It seems painful to start up a full GRPC endpoint in testing, maybe there's a mock solution?

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[Viicos]

I think we should only allow this if it is a grpc url?

Alternatively, perhaps we could split the logic, and have different code paths depending on whether the endpoint is HTTP/gRPC