feat: add aws_c5_18xlarge

[clumsy]

Adding a GPU-less AWS instance type for compute-intensive workloads, e.g. dataset pre-processing and inference.

https://aws.amazon.com/ec2/instance-types/c5

Adding only one of the high end specs, but can add others if needed.

Test plan:

================================================================== test session starts ==================================================================
platform darwin -- Python 3.12.4, pytest-7.4.4, pluggy-1.0.0
<REDACTED>
plugins: cov-5.0.0, anyio-3.7.1, typeguard-2.13.3, timeout-2.3.1, xdist-3.6.1
collected 11 items

torchx/specs/test/named_resources_aws_test.py::NamedResourcesTest::test_aws_c5_18xlarge PASSED                                                    [  9%]
...

================================================================== 11 passed in 0.07s ===================================================================