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

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

[vishalzanzrukia]

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