Support specifying integration-specific context type names.

[tachyonics]

Issue #, if available:

Description of changes: Support specifying integration-specific context type names. There are two use cases this change enables-

1. A service wants to avoid using Existential Types in its context. With this change a service can-
   i. define a generic context type[1]
   ii. a non-generic typealias[2] in its integration target
   iii. specify the non-generic typealias[3] as the contextTypeName
   iv. then use a context with concrete types at runtime, avoiding the runtime overhead of existentials
   v. specify a context with Mock concrete types[4] for testing
2. A service wants to be able to use a different context type name; for whatever reason the default currently expected by the code generator is not suitable.

[1] https://github.com/amzn/smoke-framework-examples/blob/no_existential_types_example/NoExistentialTypesExampleService/Sources/NoExistentialTypesExampleOperations/NoExistentialTypesExampleOperationsContext.swift#L35
[2] https://github.com/amzn/smoke-framework-examples/blob/no_existential_types_example/NoExistentialTypesExampleService/Sources/NoExistentialTypesExampleOperationsHTTP1/HTTPNoExistentialTypesExampleOperationsContext.swift#L24
[3] https://github.com/amzn/smoke-framework-examples/blob/no_existential_types_example/NoExistentialTypesExampleService/smoke-framework-codegen.json#L8
[4] https://github.com/amzn/smoke-framework-examples/blob/no_existential_types_example/NoExistentialTypesExampleService/Tests/NoExistentialTypesExampleOperationsTests/NoExistentialTypesExampleTestConfiguration.swift#L71

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.