Add volume_size support for gaussdb mysql

[niuzhenguo]

This adds support for specifying/updating volume size of prePaid
gaussdb mysql instance.

What this PR does / why we need it:

Which issue this PR fixes:
(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)
fixes #1166

Special notes for your reviewer:

Release note:

PR Checklist

• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

make testacc TEST='./huaweicloud' TESTARGS='-run=TestAccSomethingV0_basic'
...
=== RUN   TestAccSomethingV0_basic
--- PASS: TestAccSomethingV0_basic (70.75s)
PASS
ok      github.com/huaweicloud/terraform-provider-huaweicloud/huaweicloud       70.796s