Describe the feature
Add the ability to bind to LDAP with user's credentials, for situations where neither anonymous bind allowed nor a dedicated LDAP bind user is available.

Describe the solution you'd like
Add the %p wildcard to "Bind Password" in LDAP BindDN (similar to %s for the username) to pass the user-entered password to bind to LDAP.

Describe alternatives you've considered
I've not been able to get Gogs to connect to LDAP for our AD domain. The admins won't allow anonymous bind nor create a dedicated bind user, so I patched Gogs to pass through the user's password for use in binding to LDAP

Additional context
diff of internal/auth/ldap/config.go below:

111c111
< func (c *Config) findUserDN(l *ldap.Conn, name string) (string, bool) {
---
> func (c *Config) findUserDN(l *ldap.Conn, name string, passwd string) (string, bool) {
116c116,119
<               err := l.Bind(bindDN, c.BindPassword)
---
>               bindPW := strings.Replace(c.BindPassword, "%p", passwd, -1)
>               err := l.Bind(bindDN, bindPW)
220c223
<               userDN, found = c.findUserDN(l, name)
---
>               userDN, found = c.findUserDN(l, name, passwd)