Add RequestContext method to read attachments

[daanjanschouten]

Before this PR

Adds an attachment() method to RequestContext to read HttpServerExchange attachments. This enables retrieving current request size from Witchcraft's BytesReadHandler in application code.

RequestContext is internal-only per its javadoc, so adding methods should be safe.

After this PR

==COMMIT_MSG==
Add RequestContext method to read attachments.
==COMMIT_MSG==

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Description

Add RequestContext method to read attachments.

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✨ Features

• Add RequestContext method to read attachments. (#2771)

[pkoenig10]

I'm not sure this is appropriate for RequestContext.

RequestContext is designed to be a read-only interface for getting requests data not supported by the tool used to generate the endpoint implementations (either conjure or conjure-undertow-annotations). From the Javadoc for this interface:

Interface providing a view over data provided by the the original HTTP request including request headers and query parameters.

This method is meaningfully different. It's not exposing information about the request - it's exposing information from the underlying HttpServerExchange for this request.

[pkoenig10]

This enables retrieving current request size from Witchcraft's BytesReadHandler in application code.

I'm also not convinced that we want to make this part of the public API of Witchcraft. This increases the surface area of the Witchcraft API and removes a degree of freedom for evolving our implementation in the future if necessary.