update encode_options and decode_options typespecs and docs

[joseph-lozano]

This PR addresses
The type definitions for encode_options and decode_options were duplicated across csv.ex, encoder.ex and decoder.ex. This PR unifies the typespecs into csv.ex.

Additionally, it adds Keyword.t() as a type for :headers, as indicated by the README. It also updates the docs for encode/2 to reflect this usage.

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Checklist

• - Tests added
• - Coverage increases or stays the same
• - Docs updated
• - Changelog updated

[coveralls]

Coverage increased (+0.5%) to 100.0% when pulling 0a193c3 on joseph-lozano:fix-types-and-docs into b834446 on beatrichartz:main.

[beatrichartz]

Thanks, this is merged and released in 3.0.3