feat: Default ingestion policy mappings merged with per-tenant mappings

[salvacorts]

What this PR does / why we need it:

This PR adds support for default policy stream mappings that are automatically merged with per-tenant overrides, following the same pattern used for GlobalOTLPConfig. This allows operators to define common policy stream mappings at the distributor level that are automatically applied to all tenants, while still allowing per-tenant customizations.

With this change, operators can define default mappings once and have them automatically applied to all tenants.

How does it work?

1. Default mappings are configured in the distributor config under default_policy_stream_mappings
2. During startup, these defaults are applied to the global limits configuration
3. When loading tenant limits from YAML, the defaults are automatically merged with tenant-specific mappings
4. Tenant-specific mappings take precedence over defaults, allowing for customization

Example

Default mappings in distributor config:

distributor:
  default_policy_stream_mappings:
    finance:
      - selector: '{namespace="prod", container="billing"}'
        priority: 2
    ops:
      - selector: '{namespace="prod", container="ops"}'
        priority: 1
    security:
      - selector: '{namespace="prod", container="security"}'
        priority: 3

Per-tenant overrides in runtime config:

overrides:
  tenant1:
    policy_stream_mapping:
      finance:
        - selector: '{namespace="prod", container="custom-billing"}'
          priority: 4

Result after merging (i.e. tenant1's effective policy_stream_mapping):

tenant1:
  policy_stream_mapping:
    finance:
      - selector: '{namespace="prod", container="custom-billing"}'
        priority: 4
      - selector: '{namespace="prod", container="billing"}'
        priority: 2
    ops:
      - selector: '{namespace="prod", container="ops"}'
        priority: 1
    security:
      - selector: '{namespace="prod", container="security"}'
        priority: 3

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

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