[Python] Python HTTP signature update

Created by: sebastien-rosset

This PR is a follow up to the initial commit for HTTP signature support, based on additional testing and review.

• The OAuth2 "access_token" value should be null by default, not empty string. Otherwise, the client will attempt to set the "Authorization" header with an empty bearer token.
• Added more documentation.
• Conditionally generate the examples in the Configuration() object, based on what security schemes are defined in the OAS spec.
• In the Configuration examples, add snippet of OAS spec with a security scheme to better explain the use cases.
• Add missing import
• Fix typo

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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