Add poll plugin

[dfarr]

GET localhost:8002/default/608f05ad-115c-4943-b0ba-36be92be8fa8
data: {"href":{"claim":"...","complete":"...","heartbeat":"..."},"task":{"id":"1","counter":1,...}}

data: {"href":{"claim":"...","complete":"...","heartbeat":"..."},"task":{"id":"1","counter":2,...}}

data: {"href":{"claim":"...","complete":"...","heartbeat":"..."},"task":{"id":"1","counter":3,...}}

The polling plugin exposes a server that communicates via SSE over a persistent http connection. Once a connection is established it is left open until either:

1. the server closes the connection
2. the client disconnects

When a connection is established, the connecting process declares a group an an id by route parameters. A group is persistent and an id is intended to be an ephemeral, for example a uuid. The polling plugin sends a message to a connection, if an applicable connection is available. Messages are preferentially routed to:

1. A connection with the same group and id as the message
2. A connection with the same group, but different id; if multiple connections are possible one is chosen at random

Finally, if a connection with the same (group, id) as a pre-existing connection is established it is assumed to be an attempt to reestablish the connection and replaces the previous connection (the server will close it). This is intentionally done to mitigate the possibility that the server could maintain a connection that the client has abandoned.

[codecov]

Codecov Report

Attention: Patch coverage is 48.73905% with 1931 lines in your changes missing coverage. Please review.

Project coverage is 57.11%. Comparing base (85cf02d) to head (db42a48).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/app/subsystems/api/grpc/pb/promise.pb.go	30.24%	545 Missing and 4 partials ⚠️
internal/app/subsystems/api/grpc/pb/schedule.pb.go	35.19%	230 Missing and 2 partials ⚠️
internal/app/subsystems/api/grpc/pb/task.pb.go	34.12%	220 Missing and 2 partials ⚠️
internal/app/subsystems/api/grpc/pb/lock.pb.go	38.58%	154 Missing and 2 partials ⚠️
...ternal/app/subsystems/api/grpc/pb/callback_t.pb.go	21.79%	118 Missing and 4 partials ⚠️
...nternal/app/subsystems/api/grpc/pb/promise_t.pb.go	26.17%	109 Missing and 1 partial ⚠️
...ternal/app/subsystems/api/grpc/pb/schedule_t.pb.go	29.24%	74 Missing and 1 partial ⚠️
internal/app/subsystems/api/grpc/pb/callback.pb.go	40.00%	70 Missing and 2 partials ⚠️
cmd/config/config.go	0.00%	48 Missing ⚠️
internal/app/plugins/poll/poll.go	76.04%	37 Missing and 9 partials ⚠️
... and 30 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
+ Coverage   55.96%   57.11%   +1.14%     
==========================================
  Files         119      115       -4     
  Lines       10919    11561     +642     
==========================================
+ Hits         6111     6603     +492     
- Misses       4383     4495     +112     
- Partials      425      463      +38     

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