Improve module import

[vgvoleg]

Reduce import ydb time by ~36% (158ms → 101ms) by deferring heavy
optional dependencies that were loaded unconditionally at import time.

Changes:

• driver.py: lazy-import ydb.iam inside credentials_from_env_variables()
  instead of at module level
• iam/auth.py: move import requests and import jwt from module level
  into the functions that actually use them (MetadataUrlCredentials.__init__,
  _make_token_request, get_jwt)

Neither requests nor jwt is needed unless the user explicitly creates
MetadataUrlCredentials or calls get_jwt. Previously they were imported
on every import ydb, pulling in the full requests/charset-normalizer/
urllib3/PyJWT/cryptography stack (~60ms) even for users who never use
IAM auth.

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

[Copilot]

Pull request overview

This PR reduces import ydb time by deferring optional/heavy IAM-related dependencies until they’re actually needed, avoiding unconditional imports of the requests/PyJWT stacks.

Changes:

• Lazy-import ydb.iam in credentials_from_env_variables() instead of importing it at ydb.driver module import time.
• Move requests and jwt imports in ydb.iam.auth from module scope into the specific functions/methods that require them.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
ydb/iam/auth.py	Lazily imports jwt/requests in IAM credential helpers to avoid loading optional deps at import time.
ydb/driver.py	Defers importing ydb.iam until metadata credentials are selected, reducing baseline module import overhead.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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