password connectors: allow overriding the username attribute (password prompt)

This allows users of the LDAP connector to give users of Dex' login
prompt an idea of what they should enter for a username.

Before, irregardless of how the LDAP connector was set up, the prompt
was

    Username
    [_________________]

    Password
    [_________________]

Now, this is configurable, and can be used to say "MyCorp SSO Login" if
that's what it is.

If it's not configured, it will default to "Username".

For the passwordDB connector (local users), it is set to "Email
Address", since this is what it uses.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

Documentation/ldap-connector.md

@@ -90,6 +90,10 @@ connectors:
     bindDN: uid=seviceaccount,cn=users,dc=example,dc=com
     bindPW: password
 
+    # The attribute to display in the provided password prompt. If unset, will
+    # display "Username"
+    usernamePrompt: SSO Username
+
     # User search maps a username and password entered by a user to a LDAP entry.
     userSearch:
       # BaseDN to start the search from. It will translate to the query