Add prefix to secret key

[vinnamkim]

Summary

• From @chuneuny-emily's stress test, we observed that the DatumaroBinary format encryption integration test occasionally fails because argparse expects one argument for --encryption-key.

self = ArgumentParser(prog='datumaro_binary', usage=None, description=None, formatter_class=<class 'datumaro.cli.util.MultilineFormatter'>, conflict_handler='error', add_help=True)
status = 2, message = 'datumaro_binary: error: argument --encryption-key: expected one argument\n'

• This is because the secret key can randomly start with - and argparse recognizes it as a new argument directive.

self = ArgumentParser(prog='datumaro_binary', usage=None, description=None, formatter_class=<class 'datumaro.cli.util.MultilineFormatter'>, conflict_handler='error', add_help=True)
arg_strings = ['--encryption-key', '-VaiTYwWN41UinEp6S5ke1zPnCh3hoZajOCEVptpyt8='], namespace = Namespace(encryption_key=None)

# '-VaiTYwWN41UinEp6S5ke1zPnCh3hoZajOCEVptpyt8=' starts with '-'.

• This PR resolves this issue by adding datum- prefix to the secret key.
• Remove a redundant file also.

How to test

The existing tests cover this change.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage: 81.25% and project coverage change: +0.05 🎉

Comparison is base (fc7f607) 78.49% compared to head (ca1f6b5) 78.55%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/1.1.0     #878      +/-   ##
==================================================
+ Coverage           78.49%   78.55%   +0.05%     
==================================================
  Files                 206      205       -1     
  Lines               24939    24943       +4     
  Branches             5007     5009       +2     
==================================================
+ Hits                19577    19595      +18     
+ Misses               4236     4222      -14     
  Partials             1126     1126              

Flag	Coverage Δ
macos-11_Python-3.8	77.54% <81.25%> (+0.05%)	⬆️
ubuntu-20.04_Python-3.8	78.55% <81.25%> (+0.05%)	⬆️
windows-2019_Python-3.8	78.49% <81.25%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/components/crypter.py	80.00% <81.25%> (-0.86%)	⬇️

... and 10 files with indirect coverage changes

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