feat: options to configure chart upgrade strategy

[tthvo]

What this PR does / why we need it:

Added options (i.e. --upgrade-strategy <option>) to configure upgrade strategy for chart values. This allows maintainers to choose whichever upgrade paths align with their charts.

Without specifying the flag, default to reuse-values for backwards compatibility.

Follow up for #531

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #525

Special notes for your reviewer:

Sorry if it duplicates any in-progress work. Please let me know what you think :D