Add storage mode info

[katarinasupe]

Description

I added getter and setter for storage modes. I also updated the test setup to set storage mode to in-memory transactional by default.

Pull request type

• Feature

Related issues

Delete section if this PR doesn't resolve any issues.

Closes #239

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation @katarinasupe TODO: generate reference guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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Reviewer checklist (the reviewer checks this part)

• Core feature implementation
• Tests
• Code documentation
• Documentation on gqlalchemy/docs

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[antepusic]

We should cover the behavior of this feature when Memgraph is in a state that doesn’t allow changing storage modes (see my comment below)

[antepusic]

LGTM