Add `Sendable` conformance to `PostgresEncodingContext` (#450) #1117

Job	Run time
api-breakage	0s
gh-codeql	2m 47s
linux-integration-and-dependencies (postgres:16)	6m 12s
linux-integration-and-dependencies (postgres:14)	6m 8s
linux-unit (swift:5.7-jammy)	2m 15s
linux-integration-and-dependencies (postgres:12)	6m 4s
macos-all (postgresql@16, scram-sha-256, ~14.3)	6m 17s
macos-all (postgresql@16, scram-sha-256, ~15.0)	6m 3s
linux-unit (swift:5.8-jammy)	2m 15s
linux-unit (swift:5.9-jammy)	2m 32s
linux-unit (swiftlang/swift:nightly-5.10-jammy)	3m 5s
linux-unit (swiftlang/swift:nightly-main-jammy)	2m 57s
	46m 35s

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