Python tools for transforming segment files

[jrkinley]

This PR improves upon the segment analyser tool by adding the ability to:

• Process individual log files (not just the entire data directory)
• Fully decode record batches and write them back out in another format
• If an output path is provided then the log files are written out in JSON format by default, or a custom encoder can be given...

from segment import Segment

def my_encoder(record):
    # Encode the Record object as Avro, CSV, JSON, Proto, ...

s = Segment("/path/to/segment.log",
            dest="/path/to/output.ext"
            encoder=my_encoder)

[emaxerrno]

@ivotron and @jrkinley - we should push this to PIP - and write docs @coral-waters so the docs would say

# pip install redpanda_utils;

import redpanda_utils as rp

s = rp.Segment("path/to/file")

Great idea to make it a full package.

cc: @vsaraswat @dswang for visibility

[graphcareful]

Also maybe it would be great to add separate commits for each feature that you mentioned in your PR heading, formatting can also be a part of the workflow, i.e. every change formatted correctly

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