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return a boolean instead of an error if an extention is found on a message

Per https://github.com/ipld/specs/pull/205, metadata is now an extension, and further extensions
will be added soon

limit the size of blocks sent in a response to 512k a.l.a. bitswap

Extract metadata encoding to a package and add a decoding feature

Creates the Response Manager that processes incoming requests, initiates selector traversal, and
generates responses

add a response builder that collects components of a response and then assembles a completed
response message