support passing custom env vars for flower client

[yanchengnv]

Fixes # .

Description

This PR adds the support of passing custom env variables to Flower client. This is necessary to enable Flower client with Flare's client api to check the env var for config processing.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[YuanTingHsieh]

LGTM, I will make changes to example in another PR.