Feature: Add hooks for layout "pre" and "post"

[KoolADE85]

This PR adds new renderer hooks for users to write code that reads & updates the page layout.
In the same style as request_pre and request_post, there is now a layout_pre and a layout_post.

Contributor Checklist

• 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 follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[alexcjohnson]

💃 Looks good, just had a couple of minor comments!