Add http routing errors handler

[octo47]

This pull request adds routing error handler to address issue #1513

gRPC Gateway v2 had new error handling infrastructure added.
As a side effect of unified error handling all errors now handled as gRPC errors by standardised handler ErrorHandlerFunc.
As it works very well for gRPC errors, for cases when routing errors happen generalised error handler making some assumptions and can change expected http behaviour:

1. If no routing found gRPC error NotFound will be returned, so no way to distinguish real no handler situation from grpc method returning NotFound
2. If unexpected method requested for known handler Unimplemented grpc status returned, after conversions response will have http status code 501 (StatusNotImplemented) instead of expected 405 (StatusMethodNotAllowed) due of http -> grpc -> http status codes conversion.

To fix this PR adds additional error handler: routing error handler. Routing error handler is only called for errors related to routing problems:

• No handler -> http status NotFound
• Handler found but incorrect method -> MethodNotAllowed
• URI has incorrect format -> BadRequst

By default with no routing handler installed current behaviour will be preserved and errors will be converted to gRPC errors with following default error handler call.

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[octo47]

@googlebot I fixed it.

[johanbrandhorst]

This looks great! Could I just ask that you add a little blurb to the migration guide (https://github.com/grpc-ecosystem/grpc-gateway/blob/v2/docs/_docs/v2-migration.md) and the error handling documentation (https://github.com/grpc-ecosystem/grpc-gateway/blob/v2/docs/_docs/customizingyourgateway.md#error-handler) to tell users about this new handler? Thanks!

[octo47]

@johanbrandhorst
Updated docs!

For some reason google bot doesn't believe me I have a CLA, is it requires some time to get synced?

[octo47]

@googlebot I fixed it.

[johanbrandhorst]

The issue with the CLA bot is that your commits are made from a different email (or account) than the account you've used here. You can see it in the commit log. Could you see if you can push some new commits with the correct email?

[johanbrandhorst]

Some small wording corrections, thanks for the docs additions!

[octo47]

@johanbrandhorst thanks for suggestions! applied

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[johanbrandhorst]

@googlebot I consent.

[googlebot]

CLAs look good, thanks!

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[johanbrandhorst]

Thanks for your contribution!