[Feature][Core] Support config variable substitution with default value

[Hisoka-X]

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Description

We had supported default value with el expression feature in sink-options-placeholders.
eg:
${table_name:default_my_table}

We should support default value with el expression in all config place too. Including env, source, transform, sink.
It's a sub feature of config-variable-substitution. So the el expression with default value will be replaced with user configure value when use shell script with -i.

The flow diagram:

Usage Scenario

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Related issues

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Are you willing to submit a PR?

• Yes I am willing to submit a PR!

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[zhangshenghang]

Can be assigned to me ，thks

[Hisoka-X]

I added a flow diagram cc @zhangshenghang @arshadmohammad

[zhangshenghang]

I added a flow diagram cc @zhangshenghang @arshadmohammad

@Hisoka-X I have a general understanding of the overall idea. thks

[arshadmohammad]

Design and approach looks good to me.