feat!: Allowing custom thread pool size to configure

[vishalzanzrukia]

Fixes

Allowing custom thread pool size for users to configure for async email calls, it's totally backward compatible feature so the default thread pool size will be still the same.

Checklist

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[vishalzanzrukia]

Hi @childish-sambino @thinkingserious can someone please review this PR? thank you in advance.