python-v2: make create_facilitator_config work with both V1 and V2

[phdargen]

Description

Both v2 and v1 server should work with the same create_facilitator_config function from cdp-sdk.

• create_facilitator_config returns url + create_headers but V2's HTTPFacilitatorClient expects url+ auth_provider: updated HTTPFacilitatorClient to also accept url + create_headers

• create_headers is async but V2 client is sync: create_headers made sync in add create_facilitator_config for x402 python v2 cdp-sdk#555

• adds a iscoroutine check in legacy facilitator.py to support sync and async (for backwards compatibility) create_headers

Tests

Manually tested with modified v2 fastapi example by replacing:
facilitator = HTTPFacilitatorClient(FacilitatorConfig(url=FACILITATOR_URL)) with:

facilitator_config = create_facilitator_config(CDP_API_KEY_ID, CDP_API_KEY_SECRET)
facilitator = HTTPFacilitatorClient(facilitator_config)

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[vercel]

@phdargen is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.