* Refactor Unit Tests

* Add new certificate provider interface instead of read only certificates list.

* Update ReleaseNotes.md

* Fix Unit Tests

* Fix build

* unit testing hot swappable certificates (#1787)

* Move test classes to correct namespace

* Apply code style

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Co-authored-by: NSean Hanna <hannasm@users.noreply.github.com>

* Added certificate validation callback for .NET framework 4.x

* Update ReleaseNotes.md

* Also expose sender to event arguments

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

Only logging "Client will now throw an _MqttConnectingFailedException_." warning when actually throwing the exception (#1801)

* Warning was always logged

Warning was always logged while the text states it would throw a `MqttConnectingFailedException` exception

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

Address issue #1818 where MqttConnectionContext throws an error when passed a TcpConnection. (#1819)

* Address issue #1818 where MqttConnectionContext throws and error when passed a TcpConnection.

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

Moved check on Options.ValidateFeatures below ThrowIf methods to prevent NullReferenceException (#1800)

* Moved check on Options.ValidateFeatures below ThrowIf methods to prevent NullReferenceException

* Add Unit Test

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

This was missing from builder but not regular options.

* Return task with NULL result instead of NULL

* move Task.FromResult call out of lock

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

* Fix for websockets not applying properties unless explicitly set

* Added comment indicating why properties can't be set

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

1681 misleading signature of mqttclientconnectasync mqttclientconnectresultcode expected but throws exception (#1749)

Update MqttClient.cs

Resolve breaking change where previously ConvertPayloadToString returned null if the payload was null

* replace lock/semaphore by readerwriterlockslim to avoid lock contention while dispatch messages

* Update MqttClientSubscriptionsManager.cs

removed comment out codes

* Update ReleaseNotes.md

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Co-authored-by: NZeheng Feng <hf944813@broadcom.com>
Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

* Add new overload for options builder

* Refactor code

* Update ReleaseNotes.md

* Notify what subscription topics failed as part of the SynchronizingSubscriptionsFailedAsync callback

* Update ReleaseNotes.md

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

* ClientMessageQueueInterceptor continues with subscription processing.

* Update readme

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Co-authored-by: NChristian <6939810+chkr1011@users.noreply.github.com>

* Add compatibility option for AWS

* Fix UWP build

* Fix comment in MqttClientOptions (#1706)

* Refactor naming

* Fix wrong option value

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Co-authored-by: NMichi <michael@volz.dev>

Add user properties to event args

Fix running state handling

* Moved the usage of certificate provider into the server listener so that each connection has the posibility of receiving a new SSL cert.

* Removed warning from unused field.

* Reduced changeds on original codebase.

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Co-authored-by: NJeff Scherrer <Jeff.Scherrer@vw.com>