feat(config): Add framework for migrating deprecated plugins

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

relates to ##13375
relates to ##13376

This PR adds a framework for migrating configurations containing deprecated plugins (and options). For new the use is demonstrated for the inputs.cassandra input plugin. The PR adds a migrate sub-command to the config command.

[srebhan]

@powersj assigning this to you to get your feedback and ideas...

[powersj]

I like the general layout of this all. Thank you

One comment about documenting the boolean. Can we do test cases for the cassandra migration as well?

[powersj]

@srebhan couple errors from tests: one lint error + new test cases error

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[powersj]

will we need to a) keep this up to date with all "common" options and b) have this in each migration? or will you generalize it out after the next one?

[srebhan]

I would have loved to generalize this but I ran into the issue of TOML marshalling cannot marshal an embedded struct to the same level as the embedding struct. I.e. if you have

type A struct {
    fielda string `toml:"a"
}

type B struct {
    fieldb1 string `toml:"b1"`
    fieldb2 string `toml:"b2"`
    A
}

you will get

[[inputs.B]]
    b1 = "foo"
    b2 = "bar"
    [inputs.B.A]
      a = "fail"

instead of

[[inputs.B]]
   b1 = "foo"
   b2 = "bar"
   a = "good"

Therefore I cannot generalize it and we need to do this in every migration or fix it in the TOML marshaller...

[powersj]

One question, but should not hold up merging this.