Executable stages allow side input coders to be set and/or queried

ProcessBundleDescriptors may contain side input references from inner PTransforms. These side inputs do not have explicit coders; instead, SDK harnesses use the PCollection coders by default.

Using the default PCollection coder as specified at pipeline construction is in general not the correct thing to do. When PCollection elements are materialized, any coders unknown to a runner a length-prefixed. This means that materialized PCollections do not use their original element coders. Side inputs are delivered to SDKs via MultimapSideInput StateRequests. The responses to these requests are expected to contain all of the values for a given key (and window), coded with the PCollection KV.value coder, concatenated. However, at the time of serving these requests on the runner side, we do not have enough information to reconstruct the original value coders.

There are different ways to address this issue. For example:
* Modify the associated PCollection coder to match the coder that the runner uses to materialize elements. This means that anywhere a given PCollection is used within a given bundle, it will use the runner-safe coder. This may introduce inefficiencies but should be "correct".
* Annotate side inputs with explicit coders. This guarantees that the key and value coders used by the runner match the coders used by SDKs. Furthermore, it allows the _runners_ to specify coders. This involves changes to the proto models and all SDKs.
* Annotate side input state requests with both key and value coders. This inverts the expected responsibility and has the SDK determine runner coders. Additionally, because runners do not understand all SDK types, additional coder substitution will need to be done at request handling time to make sure that the requested coder can be instantiated and will remain consistent with the SDK coder. This requires only small changes to SDKs because they may opt to use their default PCollection coders.

All of the these approaches have their own downsides. Explicit side input coders is probably the right thing to do long-term, but the simplest change for now is to modify PCollection coders to match exactly how they're materialized.

Imported from Jira [BEAM-4271](https://issues.apache.org/jira/browse/BEAM-4271). Original Jira may contain additional context.
Reported by: bsidhom.

Start by tracing ProcessBundleDescriptors, PCollection coders, and MultimapSideInput StateRequests through runner materialization and SDK delivery. Compare the proposed approaches, then establish which change makes the coders used for materialized PCollections match those expected by side-input responses.

Written by the indexing model from the issue text.