Add support for stringArray and operationContextParams

[ashovlin]

Note for reviewers: I intentionally did not include a changelog file, as this should not result in any user-observable changes for now, but let me know if you think we should.

Issue #, if available: CLI-4648

Description of changes:

Primary Change

In the rules-based endpoints resolver, this adds support for:

1. stringArray as parameters into endpoint resolution
2. operationContextParams - this is a new trait in the service model, which can extract more complicated endpoint resolution parameters (including lists of strings) out of an operation's request.

This is a port of boto/botocore#3301.

Supporting Changes

This includes two more PRs from botocore:

1. Add support for tuples in get_attr boto/botocore#3302 - this is fixing a bug in get_attr, which is a built-in function used within endpoint resolution to resolve named keys and/or indices into arrays or dicts.
2. Reduce memory usage for scripts with many Session objects boto/botocore#2934 - this was an optimization made for boto3 to reduce memory usage when using multiple Session objects. While this is less of an issue for the CLI given a single command invocation will typically use a single Session, by overriding the LRU cache we're using for resolve_endpoint, we can support unhashable types (i.e. list, which is used for our new stringArray), which the lru_cache in functools does not.

Testing

In addition to porting the automated tests, I manually tested an internal, future service update that is taking advantage of this, and confirmed that the unit and functional tests pass locally against the updated model and endpoints files.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[hssyoo]

Just needs a new change entry.

EDIT: Oops, just saw the note in the description. I do think it's still nice to have since it makes it easier to identify the version that introduced this change.

[ashovlin]

Just needs a new change entry.

EDIT: Oops, just saw the note in the description. I do think it's still nice to have since it makes it easier to identify the version that introduced this change.

Added via 838d7c0