pageserver: add a /reset API for tenants (#6014)

## Problem

Traditionally we would detach/attach directly with curl if we wanted to
"reboot" a single tenant. That's kind of inconvenient these days,
because one needs to know a generation number to issue an attach
request.

Closes: https://github.com/neondatabase/neon/issues/6011

## Summary of changes

- Introduce a new `/reset` API, which remembers the LocationConf from
the current attachment so that callers do not have to work out the
correct configuration/generation to use.
- As an additional support tool, allow an optional `drop_cache` query
parameter, for situations where we are concerned that some on-disk state
might be bad and want to clear that as well as the in-memory state.

One might wonder why I didn't call this "reattach" -- it's because
there's already a PS->CP API of that name and it could get confusing.

test_runner/fixtures/pageserver/http.py

@@ -260,6 +260,14 @@ class PageserverHttpClient(requests.Session):
         res = self.post(f"http://localhost:{self.port}/v1/tenant/{tenant_id}/detach", params=params)
         self.verbose_error(res)
 
+    def tenant_reset(self, tenant_id: TenantId, drop_cache: bool):
+        params = {}
+        if drop_cache:
+            params["drop_cache"] = "true"
+
+        res = self.post(f"http://localhost:{self.port}/v1/tenant/{tenant_id}/reset", params=params)
+        self.verbose_error(res)
+
     def tenant_delete(self, tenant_id: TenantId):
         res = self.delete(f"http://localhost:{self.port}/v1/tenant/{tenant_id}")
         self.verbose_error(res)