fix(rds): fix rds signer cannot be used in init phase

[windmgc]

This PR fixes a problem that the rds signer cannot be used in init phase.

2023/04/17 13:11:45 [error] 784916#0: init_by_lua error: ./kong/globalpatches.lua:576: no request found
stack traceback:
        [C]: in function 'old_tcp'
        ./kong/globalpatches.lua:576: in function 'ngx_socket_tcp'
        ...ee/bazel-bin/build/kong-dev/share/lua/5.1/resty/http.lua:133: in function 'new'
        .../kong-dev/share/lua/5.1/resty/aws/service/rds/signer.lua:9: in main chunk
        [C]: in function 'require'
        ...azel-bin/build/kong-dev/share/lua/5.1/resty/aws/init.lua:467: in function 'RDS'
        ./kong/db/strategies/postgres/iam_token_handler.lua:32: in function 'init'
        ./kong/db/strategies/postgres/connector.lua:1071: in function 'new'
        ./kong/db/strategies/init.lua:37: in function 'new'
        ./kong/db/init.lua:95: in function 'new'
        ./kong/init.lua:624: in function 'init'
        init_by_lua:3: in main chunk

I believe it should because of the new() being called here since I don't really need a real socket in the rds signer. However, to keep consistent with other usages in the repo, I'll replace resty.http with luasockets.http

Tested locally inside Kong's integration.

[windmgc]

@Tieske Could you please review this? Thanks in advance!

[Tieske]

can you add a changelog entry as "unreleased"

[Tieske]

lgtm. feel free to merge after adding the changelog netry.