[Feature] Support for Polly v8 policies with backwards compatibility

[iancooper]

Is your feature request related to a problem? Please describe.
Polly, which we use for blocking retry or circuit breakers (blocking here in the sense that a lock on a queue is not released or an offset committed) now has released v8 API. If we want to support the v8API we will need to consider how to support this, as well as V7 (see https://github.com/App-vNext/Polly)

Instead of policies, we now have resilience pipelines as a target.

Describe the solution you'd like
The new resilience builder pipeline is a welcome addition, as we ourselves have had to add support for a single attribute that has multiple policies over multiple policy attributes.

So we need a version of our attribute that instead of taking a policy name, takes a resilience pipeline name. In order to retain backwards compatibility with folks that still want the v7 API we either need:

• A UseResiliencePipeline attribute that uses the V8 approach, keeping UsePolicy for v7
• Additional parameters for the UsePolicy attribute to allow setting a resilience pipeline over an array of policies

There are trade-offs to these, but the latter might be more obvious to folks.

Describe alternatives you've considered
The main alternative is to stick on v7, but v8 is more compatible with our needs, not less.

[martincostello]

Happy to help out with this migration.

FYI, you can upgrade to v8 in a non-breaking way as it retains the previous APIs (i.e. #2918 should be safe to take) - you just won't get the performance benefits of the new API in Polly.Core.

[iancooper]

@martincostello Help would be appreciated. I think we have two options. Move in v10 and make v8 style Polly a dependency, or support both syntaxes in our middleware for a bit.

[iancooper]

After a discussion with Martin we are likely to:

• Add an enumeration to the UsePolicy(Async) attribute to allow you to choose the API
• • Policy for V7
• • Strategy for V8
• Return the ExceptionPolicyHandler for V7 but the StrategyPolicyHandler for V8

[iancooper]

See also: #1197 for Polly issues for V10