gRPC Swift 1.21.0

SemVer Minor

• Add support for the Reflection service (#1654, #1659, #1669, #1675, #1677, #1680, #1682, #1694, #1695, #1699, #1703, #1709, #1712, #1731, #1732)
• Add public API to create connection IDs (#1737)

SemVer Patch

• Add some missing Sendable annotations (#1702)
• Avoid invalid state when a connect failed (#1739)
• Bump version number to 1.21.0 (#1741)