feat: check plugin init type

[tylitianrui]

Description

• check the type of plugin.init whether it is function or not. plugin.init may be defined as other data types
  in custom plugins
• remove redundant code

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[tokers]

@tylitianrui The issue link is broken, could you fix it?

[soulbird]

Why do we need this?

[tylitianrui]

check type of plugin.init. customer plugin may define it as other data types

[spacewander]

We can add it in

apisix/apisix/plugin.lua

Line 114 in 761c34a

if not plugin.priority then

and reject invalid plugin

[tzssangglass]

plugin.init may be defined as other data types in custom plugins

Frankly, I don't think it makes sense.

If init needs to be checked, then rewrite, access, etc. all need to be checked.

[github-actions]

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[github-actions]

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