**What this PR does / why we need it**:

- Allow configuring PriorityClassName for the GrafanaAgent daemonset
- Allow configuring PriorityClassName for the loki canary daemonset
without also configuring for read deployment
- Helpful when an administrator wants to prioritize logging on their
nodes

**Special notes for your reviewer**:

- Used _coalese_ style like it was already in the case of loki canary
- For loki canary, specifying on the canary takes higher precedence than
on the reader, but lower than global
- For grafana agent, global takes precedence over grafana agent to match
the existing style, but maintainers might consider flipping that around
so that most-specific is also most-prioritized

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide

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Co-authored-by: Michel Hollands <42814411+MichelHollands@users.noreply.github.com>