gRPC Swift 1.13.1

SemVer Patch

• Fix a client bug where the HTTP/2 max frame size was incorrectly set as the HTTP/2 target window size (#1528)
• Use #fileID/#filePath instead of #file (#1518)
• Make request/response parts conditionally Sendable (#1523)
• Avoid copies of large payloads (#1529, #1530)
• Delay client async writer starting (#1531)
• Bump version number to 1.13.1 (#1532)

Other Changes

• Update quick-start.md (#1527, patch credit to @compoundradius)
• Update test certs (#1522)