CDRIVER-4435 Add support for GCP service accounts in GCP KMS

[galon1]

Summary:

1. upgrade libmongocrypt dependency to 1.6.0
2. If the KMS provider is set up with an empty gcp: {}, create a gcp_service_account_token by sending a HTTP request to the GCP metadata server. Most of these changes are modeled after how azure tokens were added.
3. Testing file for service-gcp.c which uses a mock GCP server to test the code handles different HTTP responses. This test uses the fake_azure.py mock server.
4. Integration test for GCP. Adds a new task group in .evergreen/config.yml to spin up a remote GCE instance using the scripts in drivers-evergreen-tools (link to github). Here is a patch build with the new GCP KMS tasks.

Notes:

the files listed below were copied from this PR since they will be added late, but are needed for integration testing.

1. build/evergreen_config_generator/taskgroups.py
2. build/evergreen_config_lib/taskgroups.py

Most recent patch build with the new tasks can be found here: https://spruce.mongodb.com/version/6363fa95d1fe076083e2114f/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

[vector-of-bool]

Some changes requested, some about consolidating with Azure KMS work.

[vector-of-bool]

LGTM, pending one comment.

[kevinAlbs]

Nicely done. Before merging this PR, please run an Evergreen patch build with the two new tasks. The two new tasks are not run by default.
That can be done with:

evergreen patch --project=mongo-c-driver --description="Test GCP $(git log --oneline HEAD~1..HEAD)" \
    --yes --finalize \
    -v testgcpkms-variant -t testgcpkms-task \
    -v testgcpkms-variant -t testgcpkms-fail-task