Add support for `operationContextParams` Endpoints trait (#3755)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
We have to support the new [`operationContextParams`
trait](https://smithy.io/2.0/additional-specs/rules-engine/parameters.html#smithy-rules-operationcontextparams-trait)
for endpoint resolution. This trait specifies JMESPath expressions for
selecting parameter data from the operation's input type.

## Description
<!--- Describe your changes in detail -->
* Add codegen support for the [JMESPath
`keys`](https://jmespath.org/specification.html#keys) function (required
by the trait
[spec](https://smithy.io/2.0/additional-specs/rules-engine/parameters.html#smithy-rules-operationcontextparams-trait))
* Add codegen support for the trait itself. This is achieved by
generating `get_param_name` functions for each param specified in
`operationContextParams`. These functions pull the data out of the input
object and it is added to the endpoint params in the
`${operationName}EndpointParamsInterceptor`

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Updated the existing test suite for JMESPath codegen to test the `keys`
function. Updated the existing EndpointsDecoratorTest with an
`operationContextParams` trait specifying one param of each supported
type to test the codegen.

## Checklist
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rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates

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