Simplify the signing experience for timestamping

Notary Project will support Timestamping soon. Users need to specify the TSA URL when using `notation sign` command, so that timestamping countersignature can be generated for Notary Project signature envelope, for example:

`
notation sign --tsa-server "" --id "" --plugin TrustedSigning --plugin-config accountName="" --plugin-config baseUrl="" "$IMAGE"
`

I think the TSA URL is a fixed address for Trusted Signing service, so it would be easier for users to run the `notation sign` command without specifying `--tsa-server` flag. They wouldn't have to look up the URL in some documents as well.

To achieve this, it may require Notation TrustedSigning plugin to support generating signature envelope. Right now, it only supports raw signatures, so even the plugin knows the TSA URL, it is not possible for it to create a signature envelope with timestamping countersignature included.

The desired experience could be as the following:

`
notation sign --id "" --plugin TrustedSigning --plugin-config accountName="" --plugin-config baseUrl="" "$IMAGE"
`

And user can use a plugin configuration to override the default TSA server address if needed, for example the plugin configuration could be `--plugin-config tsaURL=""`