Add CLI flag --skip-tls-verify

[iwinux]

The --endpoint flag (added by #162) allows spanner-cli to connects to a custom API endpoint.

However, gRPC will only send credentials (specified by --credentials or env GOOGLE_APPLICATION_CREDENTIALS) when TLS is enabled. To ease some use cases during local testing, this pull request adds --skip-tls-verify which implicitly enables TLS while skipping certificate verification.

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[yfuruyama]

To ease some use cases during local testing

Could you elaborate on your use case where you have to skip certificate verification?

I think #162 was supposed to be used for some endpoints listed the following doc, but every endpoint uses proper TLS ceritification: https://cloud.google.com/spanner/docs/endpoints

[iwinux]

The use case involves sending Spanner queries through a local gRPC proxy (for internal logging purpose). The proxy itself is using TLS with self-signed certificate. As of security concern, the usage is not exposed to public Internet, therefore skipping verification is acceptable.

[yfuruyama]

@iwinux Thank you for sharing that. That sounds a reasonable use case.

I want to merge this, but looks like CLA is not signed yet.

[iwinux]

@yfuruyama I've signed it just now.

[yfuruyama]

@iwinux Thanks! Could you update README with a new option? https://github.com/cloudspannerecosystem/spanner-cli/blob/master/README.md?plain=1#L33-L57

[iwinux]

@yfuruyama Done

[yfuruyama]

@iwinux Thank you!