Some small Chi related tweaks

Update the README to mention Chi.

Error out if trying to generate Chi and Echo code simultaneously, as
they conflict.

Clean up security scopes a little bit

Remove unnecessary conditional in templates.

Fix oapi-codegen#40: Canonical header keys

Use the http.CanonicalHeaderKey function to normalize HTTP parameter
names.

Added a NewClientWithResponsesAndRequestEditorFunc function and updat…

…ed the RequestEditor to use a type (oapi-codegen#38)

* Added a type definition for the RequestEditor function for easy reuse

Added a new NewClientWithResponsesAndRequestEditorFunc function to be
able to pass a custom callback function during the client creation.

Use existing struct for additionalProperties that ref a schema (oapi-…

…codegen#36)

Don't create empty AdditionalProperties

When unmarshaling JSON, don't allocate the AdditionalProperties
map unless there are fields to store in it.

Skip client type generation for unsupported types

Don't generate client parsing code for unsupported types.

Refactor ClientWithResponses types

Type generation is now based off OperationDefinition and processed
schemas, so that we can do type referencing properly.

Returning the first line of the error (instead of the 'reason') (oapi…

…-codegen#31)

Improve request validation

Allow passing through openapi3filter Options and parameter
decoder, as well as passing through the echo context to inner
kin-openapi3 callbacks. This allows for implementing proper
request validation.