Shared Flags for Build and BuildRun

[otaviof]

Continue the work started on flags package to centralize all flag definitions in a single location, and using those on shp build create, shp build run and shp buildrun create, so they all rely in the same flags.

For instance:

$ shp build create --help

Creates a new Build instance using the informed first argument as its name. For example:

	$ shp build create my-app --source-url="..." --output-image="..."

Usage:
  shp build create <name> [flags]

Flags:
      --builder-credentials-secret string   name of the secret with builder-image pull credentials
      --builder-image string                image employed during the building process
      --dockerfile string                   path to dockerfile relative to repository
  -h, --help                                help for create
      --output-credentials-secret string    name of the secret with builder-image pull credentials
      --output-image string                 image employed during the building process
      --source-context-dir string           use a inner directory as context directory
      --source-credentials-secret string    name of the secret with git repository credentials
      --source-revision string              git repository source revision
      --source-url string                   git repository source URL
      --strategy-apiversion string          kubernetes api-version of the build-strategy resource (default "v1alpha1")
      --strategy-kind string                build-strategy kind (default "ClusterBuildStrategy")
      --strategy-name string                build-strategy name (default "buildpacks-v3")
      --timeout duration                    build process timeout

Additionally, improving doc-comments and getting started with long-descriptions.

[otaviof]

/retitle Build and BuildRun Create Sub-commands

[otaviof]

/cc @alicerum

[otaviof]

/retitle [WIP] Build and BuildRun Create Sub-commands

I'm going to have a look shp build run as well, make sure it's aligned.

[otaviof]

/retitle Shared Flags for Build and BuildRun

[alicerum]

/lgtm

this is awesome, thanks :D

[adambkaplan]

Mainly grammar nits and doc clean up comments. I really like the refactoring in this PR, this will help make the CLI more maintainable.

[alicerum]

/lgtm

[gabemontero]

/approve

[openshift-ci]

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