[Test Proxy] Create HTTP clients at runtime to support dynamic certificate setup

[mccoyp]

Description

Module-level, shared HTTP clients are more efficient but will be set up before SSL_CERT_DIR and REQUESTS_CA_BUNDLE are automatically set by the test proxy's setup script. This results in certificate errors when making requests to the test proxy if these environment variables aren't manually set beforehand in either a .env file or in the user's test-running environment.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[scbedd]

/azp run python - core - ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[scbedd]

• Additional Validation Builds Prior to Merge

storage
ml

[scbedd]

Gravy. Thanks for the retry on azure-ai-ml! Once that's green we should merge this.

I'm excited to see this to go in! It'll be a big simplification for all our devs.