Add an Analyzer to recommend using DIM properties with IHeaderDictionary instead of indexer

[JamesNK]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe the problem.

IHeaderDictionary has DIM properties for many commonly used headers: https://github.com/dotnet/aspnetcore/blob/ff2148be7592be83f1f88e2f372ef04f02fdfdb8/src/Http/Http.Features/src/IHeaderDictionary.Keyed.cs

Describe the solution you'd like

An analyzer that looks for indexer use with a name that matches a property.

e.g.

if (request.Headers["Content-Type"] == "application/json")
{
    // ....
}

After change (could be done via a fixer):

if (request.Headers.ContentType == "application/json")
{
    // ....
}

Additional context

No response

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We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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