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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• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[cyriltovena]

LGTM

[benclive]

LGTM!