make create-kubeconfig merge preserve some keys

[scratchmex]

this changes the behavior of create-kubeconfig merge to preserve proxy-url and namespace keys. these entries are convenient for DEX

closes #1102

changes

• preserve .contexts[].context.namespace and .clusters[].cluster.proxy-url when merging
• add unit tests for this use case
• gitignore .venv

validation

• ran

$ OCI_CLI_CONFIG_FILE=test.conf .venv/bin/python -m pytest services/container_engine/tests/unit/test_kubeconfig_merge.py -v
================== test session starts ===================
platform darwin -- Python 3.11.15, pytest-7.1.2, pluggy-0.13.0 -- /Users/ivang/code/oci-cli/.venv/bin/python
cachedir: .pytest_cache
rootdir: /Users/ivang/code/oci-cli, configfile: tox.ini
plugins: forked-1.0.2, xdist-1.22.2, cov-2.5.1
collected 2 items

services/container_engine/tests/unit/test_kubeconfig_merge.py::TestKubeconfigMerge::test_preserves_namespace_when_updating_context PASSED              [ 50%]
services/container_engine/tests/unit/test_kubeconfig_merge.py::TestKubeconfigMerge::test_preserves_proxy_url_when_updating_cluster PASSED              [100%]

=================== 2 passed in 0.01s ====================

# test.conf
[DEFAULT]
fingerprint=a:b:c
key_file=/tmp/somepath
tenancy=ocid1.tenancy.oc1..xyz
region=eu-zurich-1
security_token_file=/tmp/somepath

• ran .venv/bin/python -m oci_cli.cli ce cluster create-kubeconfig --token-version 2.0.0 --kube-endpoint PRIVATE_ENDPOINT --cluster-id ocid1.cluster.oc1.eu-zurich-1.xyz --region eu-zurich-1