chore: add retries to read path in postgres datastore impl

[tstirrat15]

Description

This was identified as a part of trying to fix a flaky test. The idea is that if the postgres driver gets a network-level error on a read, it should be retryable; we previously weren't retrying these. It should theoretically slightly improve read resiliency.

Changes

• Add retries on network errors
• Add tests for those retries

Testing

Review.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/datastore/postgres/postgres.go	78.58%	2 Missing and 1 partial ⚠️
internal/datastore/postgres/revisions.go	83.34%	1 Missing and 2 partials ⚠️

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[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 2.

Benchmark suite	Current: 1af38ab	Previous: cbdf06c	Ratio
BenchmarkDatastoreDriver/cockroachdb-overlap-static/TestTuple/SnapshotReverseRead (github.com/authzed/spicedb/internal/datastore/benchmark)	35925563 ns/op 174908 B/op 20200 allocs/op	6805173 ns/op 172708 B/op 20195 allocs/op	5.28
BenchmarkDatastoreDriver/cockroachdb-overlap-static/TestTuple/SnapshotReverseRead (github.com/authzed/spicedb/internal/datastore/benchmark) - ns/op	35925563 ns/op	6805173 ns/op	5.28
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/SnapshotReverseRead (github.com/authzed/spicedb/internal/datastore/benchmark)	32436578 ns/op 173664 B/op 20197 allocs/op	6853237 ns/op 172793 B/op 20195 allocs/op	4.73
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/SnapshotReverseRead (github.com/authzed/spicedb/internal/datastore/benchmark) - ns/op	32436578 ns/op	6853237 ns/op	4.73
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/Touch (github.com/authzed/spicedb/internal/datastore/benchmark)	6044648 ns/op 22170 B/op 287 allocs/op	2857511 ns/op 22094 B/op 287 allocs/op	2.12
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/Touch (github.com/authzed/spicedb/internal/datastore/benchmark) - ns/op	6044648 ns/op	2857511 ns/op	2.12
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/Create (github.com/authzed/spicedb/internal/datastore/benchmark)	6124342 ns/op 19543 B/op 281 allocs/op	2004369 ns/op 19414 B/op 281 allocs/op	3.06
BenchmarkDatastoreDriver/cockroachdb-overlap-insecure/TestTuple/Create (github.com/authzed/spicedb/internal/datastore/benchmark) - ns/op	6124342 ns/op	2004369 ns/op	3.06

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