This patch adds a scylla_inject_error(), a context manager which tests
can use to temporarily enable some error injection while some test
code is running. It can be used to write tests that artificially
inject certain errors instead of trying to reach the elaborate (and
often requiring precise timing or high amounts of data) situation where
they occur naturally.
The error-injection API is Scylla-specific (it uses the Scylla REST API)
and does not work on "release"-mode builds (all other modes are supported),
so when Cassandra or release-mode build are being tested, the test which
uses scylla_inject_error() gets skipped.
Example usage:
```python
from rest_api import scylla_inject_error
with scylla_inject_error(cql, "injection_name", one_shot=True):
# do something here
...
```
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Closes#12264
