HK API: allow longer timeouts for calls to backend

[Annopaolo]

Sometimes, on long operations (e.g. realm creation or deletion), synchronous calls may seem to fail because the underlying RPC to HK backend times out. Introduce the HOUSEKEEPING_API_RPC_TIMEOUT env var to allow setting a longer timeout (in milliseconds). Default to 5 seconds, as before.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a882ec1) 68.34% compared to head (45f67d9) 68.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #906 +/- ## ======================================= Coverage 68.34% 68.35% ======================================= Files 274 274 Lines 7067 7069 +2 ======================================= + Hits 4830 4832 +2 Misses 2237 2237 ```

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