Define additionalProperties in stream maps config

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/sdk/-/issues/324

Originally created by @aaronsteers on 2022-02-04 00:02:31

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The following discussion from !236 should be addressed:

• [ ] @edgarrmondragon started a discussion: (+3 comments)

  @aaronsteers Wdyt about defining additionalProperties here so the expected value types are validated?

  For example: https://github.com/MeltanoLabs/meltano-map-transform/blob/fc779f8664e4d9255f6c5d8a5ee1321f84f3f014/meltano_map_transform/mapper.py

[MeltyBot]

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[edgarrmondragon]

This is an easy one if anyone's interested in taking it: https://github.com/MeltanoLabs/meltano-map-transform/blob/fc779f8664e4d9255f6c5d8a5ee1321f84f3f014/meltano_map_transform/mapper.py#L19-L40