let add command get JWT from a file like the enroll command

[qrkourier]

This is awkward to document and to use.

sudo ziti-edge-tunnel add --jwt "$(< ./in-file.jwt)" --identity myIdentityName

This would be nicer.

sudo ziti-edge-tunnel add --jwt ./in-file.jwt --identity myIdentityName

If stdin too were available then the add command would behave like the enroll command's --jwt option.

sudo ziti-edge-tunnel add --jwt - --identity myIdentityName < ./in-file.jwt

[qrkourier]

This will improve the user experience and simplify this doc: https://openziti.io/docs/reference/tunnelers/linux/#add-a-single-identity