ldap: modify variable name and initialize connection with root_dn and root_pwd (#43824)

[ti-chi-bot]

This is an automated cherry-pick of #43824

What problem does this PR solve?

Issue Number: close #43822

Problem Summary:

1. TiDB uses variable root_pw, which is different from root_pwd.
2. TiDB doesn't support the server which doesn't allow anonymous bind.

What is changed and how it works?

1. Rename the variables to root_pwd.
2. Initialize the connection with binding to the root_dn and root_pwd.

The only concern is that the the ACL configuration for anonymous user and root could be different.

Check List

Tests

• Unit test
• Integration test
• Manual test
• No code

I manually tested this patch with okta ldap service, which doesn't support anonymous binding.

Copied from #43822, I also attached the "Alternatives" here:

I have checked the behavior on two different implementations:

Alternatives

1. Percona server, as shown in its code, will also use root_dn and root_pwd to initialize the connection.
2. MySQL server doesn't detect the connection status. If a connection is killed (or lost) from the server side, the connection will pollute the connection pool, and all following login will fail. (tcpdump on the LDAP server, no request will arrive).

I think the first one is apparently better.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CbcWestwolf
• bb7133

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[bb7133]

LGTM

[bb7133]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 1f5a013