These two streams mostly play together. The former provides an input_stream from read from in-memory temporary buffers, the latter wraps it to limit the size of provided temporary buffers. Both are used to test contiguous data consumer, also the buffer_input_stream has a caller in sstables reversing reader.
This PR removes the buffer_input_stream in favor of seastar memory_data_source, and moves the limiting_input_stream into test/lib.
Enanching testing code, not backporting
Closesscylladb/scylladb#28352
* github.com:scylladb/scylladb:
code: Move limiting data source to test/lib
util: Simplify limiting_data_source API
util: Remove buffer_input_stream
test: Use seastar::util::temporary_buffer_data_source in data consumer test
sstables: Use seastar::util::as_input_stream() in mx reader
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