suggestion: add system wide amalgamated zstd.(h|c) and use in dtpc optionally for adu compression

[zebastian]

In general its the applications layer responsibility compressing payload before handing to ION-DTN parts.

Anyway there are quite some parts which could benefit from zstd inside ION-DTN:

• dtpc adu
• compression of bundle sdrs on disk
• cfdp file transfer
• (bp)
• ...