Add resize support for FlexVolume

[xingzhou]

Feature Description

• One-line feature description (can be used as a release note): Add resize call support for FlexVolume to support volume resizing like LVM expansion
• Primary contact (assignee): Tom Xing(@xingzhou)
• Responsible SIGs: sig-storage
• Design proposal link (community repo): Proposal for growing FlexVolume size. community#1700
• Reviewer(s) - (for LGTM) recommend having 2+ reviewers (at least one from code-area OWNERS file) agreed to review. Reviewers from multiple companies preferred: @gnufied & @chakri-nelluri
• Approver (likely from SIG/area to which feature belongs): TBD
• Feature target (which target equals to which milestone):
  • Alpha release target 1.13
  • Beta release target TBD
  • Stable release target TBD

[xingzhou]

According to the discussion in kubernetes/community#657, we want to add volume resize support for FlexVolume, including:
• Improve FlexVolume to support volume resizing call
• Support LVM volume expansion
/cc @gnufied, this feature request depends on #284

[gnufied]

Also we would want @chakri-nelluri as reviewer. Because he is primary author of flex.

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[xingzhou]

We are starting to write a proposal doc first at:
https://docs.google.com/document/d/1dwom9xQ3Fg5F_jJrybr0slp-QsO3_CKiisxzNRoMhec/edit?usp=sharing

will update the progress then

[chakri-nelluri]

Thanks @xingzhou.
Reviewed and added comments.

[xingzhou]

Thanks @chakri-nelluri for the review, have responded with my answers, please take a look.

@saad-ali and @idvoretskyi, we are planning this feature for v1.10, would you please help to add this ticket to this milestone? thanks