Proposal: Advanced LDAP authentication plugin

# Background

Currently APISIX has provided a simple LDAP authentication plugin that supports simple bind functions, but does not support, for example, user groups, for which users have a need. We can do this by introducing a new plugin to provide a platform to host these newly added and future needed features.

# Benefits

Users can use the new plug-in to bind identity groups to consumers, and colleagues can also use consumerless mode (i.e., authentication only). In the future, the plug-in can be extended to support more complex authentication models.

# Goals

Create a new plugin, called `ldap-auth-advanced`.

## Configuration design

### Route configuration

Name | Type | Optional | Default | Description
-- | -- | -- | -- | --
ldap_uri | string | no |   |  
use_starttls | boolean | yes | false |  
use_ldaps | boolean | yes | false |  
ssl_verify | boolean | yes | true |  
timeout | integer | yes | 10000 | socket timeout time
keepalive | boolean | yes | yes | Whether to enable connection pool
keepalive_timeout | integer | yes | 60000 | Connection pool connection timeout time
keepalive_pool_size | integer | yes | 5 | Connection pool size
keepalive_pool_name | string | yes | : | Connection pool name
ldap_debug | boolean | yes | false | Record search logs
consumer_required | boolean | yes | true | Whether the consumer must attach it or not, the certification will fail.
user_dn | string | no |   | DN template for identifying users (cn=%s,ou=users,dc=example,dc=org)
user_membership_attribute | string | no | memberOf | Add: Used to indicate which attribute in a user object indicates the group to which it belongs. For example memberOf.

### Consumer configuration

Name | Type | Optional | Default | Description
-- | -- | -- | -- | --
user_dn | string | yes |   | User DN, mutually exclusive
group_dn | string | yes |   | Group DN, mutually exclusive

## Plugin logic

1. User access triggered route's access phase
2. The plugin uses `lua-resty-ldap` to connect to the LDAP server. (Connection reuseable)
3. Execute the binding according to the configuration.
4. Performs a search based on the user dn to fetch `user_membership_attribute`
5. Try to attach user dn or group dn to consumer (user dn > group dn)

### Tips

- The plugin allows the use of route + consumer and pure route modes.
- Consumer support for mapping individual users or individual groups.
- The plugin runs in the access phase.
- The plugin maintains its own connection pool (specified configuration).

# Other

I would like to hear your opinions and welcome comments.