bug: plugin basic-auth needs required field

[juzhiyuan]

Issue description

Plugin basic-auth doesn't have required field currently.

Environment

• apisix version (cmd: apisix version):
• OS:

Minimal test code / Steps to reproduce the issue

None

What's the actual result? (including assertion message & call stack if applicable)

No required field.

What's the expected result?

It should have at least one required field such as username or password in my opinion.

[membphis]

For all of the authorization plugin:

When it was used with consumer, it is necessary to provide specific authentication information.

When it was used with route / service, we don't need any parameters, just enable or disable this plugin.

So I think the current way is right.

How about this schema definition? Is it better?

oneof = {
    -- no parameter
    required = {}, 
    -- two parameters
    required = {"username", "passwd"}
}