The v2 AWS SDK introduces a new endpoint resolution mechanism where
custom endpoints are specified on a per-service basis, while the
previous global mechanism is deprecated. This changeset switches to the
newer mechanism.

The default resolver under the new mechanism uses the base endpoint in
a service client's `Options` as the means to customize endpoints, so
it's enough for tests to make sure the base endpoint gets set.

Under the previous mechanism, the CHAMBER_AWS_SSM_ENDPOINT environment
variable was used to customize not just the endpoint for SSM, but also
for Secrets Manager. This changeset deprecates the latter use in favor
of a new CHAMBER_AWS_SECRETS_MANAGER_ENDPOINT environment variable.
There seems to also be SDK-native ways to use the environment for
endpoint customization, so this whole thing might go away at some point.