feat(modify): new modifier add_header

[msessa]

Partially addresses #158

Description

This PR adds a new modifier add_header which allows arbitrary headers to be added to a message in the pipeline.

Use Case

A practical use case is to inject the X-SES-FROM-ARN header before forwarding the message to AWS SES to control which identity will be used to verify the sender domain.

See reference here

[foxcpp]

Overall LGTM. Can you add a configuration option to not return an error on duplicate header? It is correct to have multiple values for some header fields and such configuration option may be useful for some cases. I would probably make it default even as "least surprise" principle.

[msessa]

@foxcpp thanks for you review (and for creating maddy!)

I didn't realize the spec permitted fields appearing multiple times.
I've adapted the code and tests for that

[msessa]

btw I'm happy to also implement other variations of the modifier, eg: upsert_header remove_header if you think they could be useful

[msessa]

@foxcpp any update on this PR?

[DEVisions]

Interested in this feature as well. Hope it gets merged :)