Fixed#24296
While the HTTP client used for REST calls in AWS/GCP KMS integration (EAR)
is not general enough to be called a HTTP client as such, it is general
enough to be called a REST client (limited to stateless, single-op REST
calls).
Other code, like general auth integrations (hello Azure) and similar
could reuse this to lessen code duplication.
This patch simply moves the httpclient class from encryption to "rest"
namespace, and explicitly "limits" it to such usage. Making an alias
in encryption to avoid touching more files than needed.
Closesscylladb/scylladb#24297
Refs #22925
Adds some wrapping and helpers for the kind of REST operations we
expect to perform.
Some things like stream formatting is redundant visavi seastar,
but on that level we only have \r\n encoded writing to
output_stream and similar, which is less useful for things like
logging.
