Only rate limit ACR referrer lookup requests

[lbussell]

This PR fixes #2141. The issue text contains the detailed explanation of why these changes are necessary, but summarizing here:

• The previous rate limiting changes, Rate limit HTTP requests to Azure Container Registry APIs #2137 and Bound concurrent ACR requests and decouple ACR rate limiting from timeouts #2139, were too restrictive and slowed down publishing too much - it was an order of magnitude slower than the theoretical lowest ACR rate limit due to unnecessarily tight parallelism limits.
• ACR referrer lookups have a much lower rate limit than the rest of the API.
• Therefore, this PR reverts parallelism limits and applies rate limiting to only the ACR referrer lookup through OrasDotnetService using a named HttpClient.
• Http resilience changes from the previous PRs remain in place.