[Backport 2024.2] feature(sct_config): appendable configuration values

[mergify]

introduce new option for appending to string or list configuration values

1. strings: can be appended with adding ++ at the begining of the string:
   export SCT_APPEND_SCYLLA_ARGS="++ --overprovisioned 1"

2. list: can be appended by adding ++ as the first item of the list
   export SCT_SCYLLA_D_OVERRIDES_FILES='["++", "extra_file/scylla.d/io.conf"]'

Note list would work on every config defiend with str_or_list_or_eval with the expection of config_files, region_name, gce_datacenter and gce_datacenter which can't be appended, and would need to be overriden completly

Ref: #7653

Testing

• [ ]

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit e80f2bf)

Parent PR: #9554

[mergify]

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Changes to be committed:
	modified:   unit_tests/test_teardown_validator.py

Unmerged paths:
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	both modified:   docs/configuration_options.md
	both modified:   sdcm/sct_config.py
	both modified:   unit_tests/test_config.py

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