Add OpenAPI operation and schema transformer interfaces

[martincostello]

Add OpenAPI operation and schema transformer interfaces

Add interfaces for transforming OpenAPI operations and schemas.

Description

• Add IOpenApiOperationTransformer.
• Add IOpenApiSchemaTransformer.
• Rename Use*Transfomer() methods to Add*Transformer().

Fixes #56022

[martincostello]

Test failure is #56372.

[captainsafia]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 3 pipeline(s).

[captainsafia]

Test failures seem to be an instance of https://github.com/dotnet/aspnetcore/issues/56828 that is affecting a few other PRs. The particular failrue doesn't seem like a flake but it's not impact all PRs so there might be something here...