Fix error with LuigiTomlParser usage in S3Client

[junhl]

Description

My attempt to address issue of toml usage error in S3Client from #2996

I'm providing two solutions to choose from, one to fill lacking functions into LuigiTomlParser and another to change the usage into more generic one. While I prefer the second one, when the reviewers prefer one over another, I'll add tests for it.

LuigiConfigParser is built on top of Python's ConfigParser, which is MutableMapping at the end.
LuigiTomlParser is using toml (the module), and it does not provide a comparable class but rather provide load/dump via decoder/encoder.
They work a bit differently. A touch to BaseParser may be needed on a long term because there may be other issues due to differences in their functionalities (not being addressed in this PR).

Motivation and Context

Resolves error on S3Client when attempting to use toml. It would give this before:

defaults = dict(configuration.get_config().defaults())
AttributeError: 'LuigiTomlParser' object has no attribute 'defaults'

Have you tested this? If so, how?

I ran my jobs with this code and it works for me.

I'm happy to add tests when one solution is preferred over another by the reviewers.

[junhl]

solution 1: fill lacking functions to LuigiTomlParser. This is just dummy function so it's not my preferred one.
(will add tests if preferred by reviewers)

[junhl]

solution 2: change usage to generic one to use existing __getitem__ and only take defaults when it's meaningful (ex. LuigiConfigParser) instead of creating dummy function. (my pref. Will add tests if reviewers also think so)

[stale]

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