Use map[string]string instead of []string internally

[mcanevet]

What does this PR do?

This PR uses a map[string]string instead of a []string in ̀action.go` so that we have a more flexible structure to output the secrets in different ways (see #184).
For example we could easily have:

• quote multi-line secrets in @SUMMONENVFILE (if we find an agreement on this)
• have @SUMMONJSONFILE or @SUMMONYAMLFILE magic strings like @SUMMONENVFILE

Checklists

Change log

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Test coverage

• This PR includes new unit and integration tests to go with the code changes, or
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Documentation

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[diverdane]

@mcanevet,
Thank you for this contribution! This makes a lot of sense, keeping keys and values separate until they absolutely have to be combined. This will probably makes things a little more extensible for adding enhancements/features to Summons actions.