Refactor OmegaConfigLoader

[merelcht]

Description

The methods in OmegaConfigLoader are big and getting more complicated each time we make a change.

Development notes

I simplified things by:

• Extracting the filesystem and protocol setup into a separate function _initialise_filesystem_and_protocol
• Extracting the merging steps of the base and default_run_env config into a separate function _merge_configs
• Added a check to return early if chosen env config is the same as base config to avoid loading the same config twice and not do unnecessary merging.

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[ElenaKhaustova]

Thank you, @merelcht, changes looks good to me.

Left one non-blocking suggestion.

[ElenaKhaustova]

Shall we go even further and move merging strategies outside to the Enum and create a mapping of symbolic names to the implementations? Then all the possible values and implementations will be in one place, so it's easier to navigate and compare them, and additionally, we will get rid if else logic when applying a merging strategy.

[ElenaKhaustova]

Some clarification on the above comment.

I was thinking of something like that:

class MergingStrategies(Enum):
    FIRST = "first"
    SECOND = "second"

def first_strat():
    pass
    
def second_strat():
    pass
    
merging_strategies_map = {
    MergingStrategies.FIRST: first_strat,
    MergingStrategies.SECOND: second_strat,
}

Usage:

first = strategies_map[MergingStrategies.FIRST]()
second = strategies_map[MergingStrategies("second")]()

[ElenaKhaustova]

Thank you, @merelcht, all clear! ✨

[ankatiyar]

Looks good! 💯