sr: add the base16 RSA identity digest to commit

sr: add the base16 RSA identity digest to commit

Keep the base16 representation of the RSA identity digest in the commit object
so we can use it without using hex_str() or dynamically encoding it everytime
we need it. It's used extensively in the logs for instance.

Fixes #19561

Signed-off-by: David Goulet <dgoulet@torproject.org>