[WIP] Make CSRF header configurable

[mallwang]

This PR solves any issues with the hardcoded CSRF header (see #729) added by the Dash Renderer by making the header name and cookie name configurable. Both values can be passed as a hook object to the DashRenderer instance.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • add new hook for setting a config object for the CSRF header name and cookie name
  • set the previous header name and cookie name as default values
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follow
  • this github https://github.com/plotly/dash-docs/pull/684 updates the dash docs