fix(ruler): validate remote write config (backport k281)

[loki-gh-app]

Backport e916944 from #19920

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What this PR does / why we need it:

remote write config validation is missing in a few places. This also ensure that all relabling config have a valid NameValidationScheme set.

ruler panic: Invalid name validation scheme requested: unset
ruler
ruler goroutine 1264 [running]:
ruler github.com/prometheus/common/model.ValidationScheme.IsValidLabelName(0xc0054d5a7a?, {0xc000d24008?, 0x8?})
ruler     /src/enterprise-logs/vendor/github.com/prometheus/common/model/metric.go:203 +0xb4
ruler github.com/prometheus/prometheus/model/relabel.relabel(0xc00065fdc0, 0xc0008b3dc0)
ruler     /src/enterprise-logs/vendor/github.com/prometheus/prometheus/model/relabel/relabel.go:340 +0x4c5
ruler github.com/prometheus/prometheus/model/relabel.ProcessBuilder(...)

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

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
• 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

[ashwanthgoli]

lgtm