Document Snowflake connector proxy configuration and support proxy connection options

[ChristineTChen]

What does this PR do?

Documents environment variable approach: https://docs.snowflake.com/en/user-guide/python-connector-example.html#using-a-proxy-server

Supports connection proxy params: https://github.com/snowflakedb/snowflake-connector-python/blob/master/src/snowflake/connector/proxy.py#L10

Fixes #8019

Motivation

FR

The previous wrapper workaround is error prone. This PR uses the Snowflake Connector method (originally avoided because it automatically sets proxy env vars as well).

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #8181 (01417af) into master (84eb484) will increase coverage by 5.69%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
snowflake/datadog_checks/snowflake/check.py	92.75% <100.00%> (+3.46%)	⬆️
snowflake/tests/test_unit.py	97.22% <100.00%> (-0.63%)	⬇️
disk/tests/conftest.py
airflow/tests/test_unit.py
exchange_server/tests/common.py
network/datadog_checks/network/__init__.py
coredns/datadog_checks/coredns/__about__.py
druid/datadog_checks/druid/druid.py
hazelcast/datadog_checks/hazelcast/__init__.py
kube_dns/datadog_checks/kube_dns/__about__.py
... and 865 more

[ruthnaebeck]

Docs review

[ruthnaebeck]

👍 for docs