[Bug] SinkConfig.InputSpecs param dont't work.

#### Expected behavior

Although the InputSpecs parameter is included in the configuration, but pulsar-admin-go does not handle it.
https://github.com/streamnative/pulsar-admin-go/blob/b31ddd22d16b9ebd4a313381d8ed8155a0ba91cb/pkg/utils/sink_config.go#L43

#### Actual behavior

Tell us what happens instead

#### Steps to reproduce

How can we reproduce the issue

#### System configuration
**Pulsar version**: x.y

Start at pkg/utils/sink_config.go around line 43, where the issue says InputSpecs is not handled. Trace how SinkConfig parameters are processed and verify the behavior with a configuration that includes InputSpecs. Done means pulsar-admin-go handles the parameter instead of ignoring it.

Written by the indexing model from the issue text.