Fix Zstandard metadata decompression for frames without declared content size

[sr-rossanderson]

Patchman can fail when processing YUM/DNF repository metadata compressed with Zstandard where the frame does not expose a known decompressed content size.

This was encountered against modern EL9 repository metadata.

Current behaviour:

could not determine content size in frame header

Expected behaviour:

Patchman should be able to decompress valid Zstandard repository metadata regardless of whether the frame declares its decompressed content size.

Suggested fix:

Use streaming decompression instead of an API which requires the output size to be known, for example:

from io import BytesIO
import zstandard

dctx = zstandard.ZstdDecompressor()

with dctx.stream_reader(BytesIO(contents)) as reader:
    decompressed = reader.read()

Impact:

This prevents Patchman from consuming some current EL8/EL9-style repository metadata.