feature: Header and trailer checks

[evaline-ju]

Feature Description

Currently the external processor server that leverages plugins only focuses on request/response bodies. If plugins are to be run on request and response headers and trailers, the server has to be updated.

Proposed Solution

Processor implementation has to run plugins on request/response trailers/headers as appropriate

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Additional Context

No response

[julianstephen]

@evaline-ju For req/res bodies, do we also want on-any-request calls as opposed to on-tool-invoke calls?

[evaline-ju]

@julianstephen I'm not particularly familiar with the difference (can we add a link?) but I think generally if we're able to, we want to be able to check any MCP request and response bodies. I wasn't quite sure if there's available payloads that would be general enough to allow this, while still allowing us to easily determine which Nemo guardrails calls to make