botocore: add bedrock genai user events and lazy initialize tracer

[xrmx]

Description

This starts implementing genai events support for botocore's bedrock extension. This only implements the user message as a start because they are common to all the APIs and there's already enough code to review.
This includes code to setup different tracers and event loggers (and so schema versions) depending on the extension.
This also removes the pass of AWS authentication environment variables because of security concerns, instead I've added documentation on how to override the tox configuration when needed for recording calls.

Refs #3210

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox -e py310-test-instrumentation-botocore

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[codefromthecrypt]

good I like starting small. despite lazy init being in the same change it makes sense as it shows how it is used as well (for events specifically)

[karthikscale3]

looks good