[BEAM-11104] Add ProcessContinuation type to Go SDK

[jrmccluskey]

Adds the ProcessContinuation interface to the Go SDK and adds handling for it to be a valid return value in funcx/fn.go. Also adds a unit test case for the new return type.

Design Doc: https://docs.google.com/document/d/1_JbzjY9JR07ZK5v7PcZevUfzHPsqwzfV7W6AouNpMPk/edit?usp=sharing

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Codecov Report

Merging #17265 (682a8ff) into master (4213f9e) will decrease coverage by 0.06%.
The diff coverage is 61.90%.

@@ Coverage Diff @@## master #17265 +/- ##
==========================================
- Coverage 74.16% 74.09% -0.07% 
==========================================
Files 675 679 +4 Lines 88896 89087 +191 ==========================================
+ Hits 65926 66010 +84 - Misses 21835 21926 +91 - Partials 1135 1151 +16 

Flag	Coverage Δ
go	50.06% <61.90%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/funcx/fn.go	58.10% <38.46%> (+0.56%)	⬆️
sdks/go/pkg/beam/core/sdf/continuation.go	100.00% <100.00%> (ø)
sdks/go/pkg/beam/core/runtime/graphx/dataflow.go	53.93% <0.00%> (-1.39%)	⬇️
sdks/go/pkg/beam/core/runtime/exec/coder.go	56.46% <0.00%> (-0.47%)	⬇️
sdks/go/pkg/beam/core/runtime/graphx/coder.go	53.72% <0.00%> (-0.39%)	⬇️
sdks/go/pkg/beam/core/typex/fulltype.go	41.83% <0.00%> (-0.34%)	⬇️
sdks/go/pkg/beam/core/runtime/harness/harness.go	10.96% <0.00%> (-0.30%)	⬇️
.../go/pkg/beam/core/runtime/harness/worker_status.go	51.16% <0.00%> (ø)
sdks/go/pkg/beam/core/sdf/lock.go	0.00% <0.00%> (ø)
sdks/go/pkg/beam/core/graph/coder/nil.go	59.09% <0.00%> (ø)
... and 3 more

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[jrmccluskey]

Not sure what fmt is catching here, it's not a file I touched and running fmt on the offending file (core/typex/special.go) is not changing anything.

[jrmccluskey]

PR to fix the fmt break at #17266

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[jrmccluskey]

assign set of reviewers

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R: @damccorm for label go.

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[github-actions]

R: @lostluck for final approval

[damccorm]

The bot was a little overeager, happy to have @lostluck weigh in though :) I put up a one liner fix for the bot issue fwiw

[lostluck]

Please add a link to the design doc in the top description. Makes things a bit more accessible than indirecting to the JIRA (which isn't always as discoverable as one might hope).

[lostluck]

Pretty good TBH. A few comments/changes on this pass.

[lostluck]

LGTM at this point, other than the suggested edits. Errors don't have to be pithy and short. We can add more information, like links to the jira, which when resolved will indicate which release starts supporting the feature.

[lostluck]

Thanks!