feat: support custom variables_hash_max_size parameters in config.yaml

[Firstsawyou]

What this PR does / why we need it:

Signed-off-by: Firstsawyou yuelinz99@gmail.com

When apisix is ​​in make run, it will prompt the following warning message:

$ make run
./bin/apisix start
/usr/local/openresty/luajit/bin/luajit ./apisix/cli/apisix.lua start
nginx: [warn] could not build optimal variables_hash, you should increase either variables_hash_max_size: 1024 or variables_hash_bucket_size: 64; ignoring variables_hash_bucket_size

Therefore, this PR will add the variables_hash_max_size parameter to solve the above warning problem.
Since the value of variables_hash_max_size: 1024 is not enough, set variables_hash_max_size: 2048

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