Allow pass maven artifact through flags #256

[shashwathi]

What this PR does / why we need it

Allows passing maven info through flags.

• If flags are set, these will take priority over paramsyaml
• maven source can be updated partially (no need for all the flags) only through flags, if just one of the items is to be updated, don't use paramsyaml since it's going to delete the ones that are not updated

Which issue(s) this PR fixes

Fixes #170

[shashwathi]

/rebase

[anibmurthy]

lgtm

[shaheerkootteeri]

A clarification in the code

[heyjcollins]

AC

1. confirmed presence and accuracy of --maven* flags and descriptions in tanzu apps workload create/update/apply help output
2. confirmed maven-based workload creation via --params-yaml hasn't regressed
3. confirmed presence of >= 1 --maven* flag overrides --params-yaml completely
4. confirmed validation is taking place for the 3 required maven flags (--maven-artifact, --maven-group, --maven-version)
5. confirmed it's possible to execute a partial update of an existing maven-based workload (can pass in less than all the required maven flags, pass validation, and just update the workload with the changed maven value(s)

LGTM overall!

Note: if all the required maven flags are provided and --maven-type isn't included, the plugin still adds spec.params.maven.type: null to the workload object. This isn't expected, however, the current backend will accept this without error so the current implementation is acceptable and we'll clean that up as a follow up change (will log a separate issue).