allow specifing grpc timeout rather than hardcoded 5 seconds

[tedli]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Introduce a optional field to config timeout of grpc calls. / The grpc provider impl may call other api which operates longer than 5s, resulted in context deadline exceeded.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

No.

externalgrpc: timeout of grpc calls could be specified through `cloud-config`

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[dbonfigli]

Looks good and I think it could be useful.

Can you please also update the readme with the new configuration parameter?

[dbonfigli]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dbonfigli, tedli

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