parse stale querystring parameter value as boolean

[tgross]

The stale parameter on the Nomad HTTP API determines the consistency mode for query endpoints. If the stale flag is set, a request to a follower will be served without being forwarded to the leader. Unfortunately the API server tolerates this flag have a value, but it only acts on the flag being set.

This means a request like /v1/jobs?stale=false is treated as a stale query. While the documentation is straightforward on this, it's bad ergonomics and can lead to mistakes. Instead, we should parse any value as a boolean (and keep the existing behavior where if the flag is set without a value, the query is stale). So in other words we want:

querystring	is stale query?
?stale	yes
?stale=true	yes
?stale=false	no

[dttung2905]

If you don't mind, please allow me to work on this issue.

[github-actions]

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