Update docker tag creation to accept image digests

[grace-rehn]

Background

Docker image references support a content digest suffix (name:tag@sha256:<hex>) for pinning to an exact immutable image. The DockerTag type has no support for this notation.

Results

• Added Digest property to DockerTag
• Added SplitDigest method that strips the @sha256:... suffix before parsing
• VersionFactory.CreateDockerTag now calls SplitDigest first, then constructs a DockerTag with the digest attached
• ToString() reconstructs the full reference including the digest when present
• Existing inputs without a digest are unaffected - Digest is null and all existing behaviour is preserved

Changes in Calamari and Octopus Server which use these new Versioning changes:
OctopusDeploy/Calamari#2055
https://github.com/OctopusDeploy/OctopusDeploy/pull/44856

Fixes HPY-1502

Before

Docker tag only supports name:tag format

After

Docker tags now support name:tag or name:tag@sha256:<digest>

Reducing risk

More Info: https://github.com/OctopusDeploy/OctopusDeploy/pull/44856

Test Instance: https://grace-image-digests.testoctopus.app/app#/Spaces-1/projects/test-image-digests/deployments/releases/0.0.5/deployments/Deployments-21?element=0%2FServerTasks-62_ER1BD1R8JP%2Fc4867cee3f10446992fb1528fbf17d22&activeTab=taskLog