Mention mysql2 pooling option in the docs

[songpr]

Prerequisites

• I have written a descriptive issue title
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🚀 Feature Proposal

Mysql2 pooling is good for performance https://www.npmjs.com/package/mysql2#using-connection-pools also it have promise pool

Motivation

To improve performance when it have a lot of connection to mysql

Example

No response

[Eomm]

It is already doable and the default setting as well

from docs:

If you don't use the connectionString option, the options that you pass to register will be passed to the MySQL pool builder.

fastify-mysql/index.js

Line 67 in e492e3c

client = mysql.createPool(options.connectionString || options)

Would you mind improving our docs?