feat: convert WalSegmentWriter to io.ReadSeeker

[vlad-diachenko]

What this PR does / why we need it: Implemented logic to convert WalSegmentWriter to a reader that implements io.ReadSeekCloser that satisfies the type required by ObjectClient.PutObject() function.

Special notes for your reviewer: Benchmark results:

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/v3/pkg/storage/wal
BenchmarkWrites
BenchmarkWrites/WriteTo
BenchmarkWrites/WriteTo-12               330       3546897 ns/op       45038 B/op        229 allocs/op
BenchmarkWrites/Reader
BenchmarkWrites/Reader-12                333       3507544 ns/op       51875 B/op        232 allocs/op
PASS

Checklist

• [ ] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [ ] Tests updated
• [ ] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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