feat(cmd): add 'update' subcommand for build resources

[mohit-bhandari45]

Changes

Introduced the missing update subcommand for build resource manager (shp build update):

• shp build update <name> [flags]: Allows updating existing Build object specifications (e.g. --source-git-url, --source-git-revision, --output-image, --strategy-name, --source-context-dir, --dockerfile, --builder-image, etc.) directly from the CLI.
• Selective Field Updates: Inspects which CLI flags were explicitly provided by the user using Cobra's Flags().Changed(...) to only modify specified fields without overwriting un-flagged spec fields.
• Error Handling: Gracefully handles missing builds with a user-friendly Build 'xyz' not found in namespace 'default'. error message.
• Unit Tests: Added comprehensive unit test suite in pkg/shp/cmd/build/update_test.go covering success and NotFound cases (100% passing).

Related Issue

Fixes #411

Type of PR

/kind feature

Submitter Checklist

• Includes tests if functionality changed/was added
• Includes docs if changes are user-facing
• Kind label has been set
• Release notes block has been filled in, or marked NONE

Release Notes

Added `shp build update <name>` subcommand allowing users to modify existing Build resource specifications directly from the CLI.

[openshift-ci]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[mohit-bhandari45]

@kaizakin Please do request a review, once you get time.