[gcloud-sqlproxy] add extraFlags (single-hyphen arguments) for command

[rverenich]

What this PR does / why we need it: At the moment --arg1=123 is possible to pass via extraArgs. additional extraFlags added for possibilities to configure with flag-like -arg1 -arg2

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

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] Chart Version bumped, is mandatory for any chart chnages
• [x] Variables and other changes are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [gcloud-sqlproxy])

[rverenich]

@rimusz , @naseemkullah , please take a look Thanks in advance