easy access sni TLSSettings smart constructor

[tmcl]

This implements my suggestion in our discussion in #859

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[kazu-yamamoto]

@tmcl Would you cherry-pick 06b3dd3 to this PR and update the reason for the pragma?

[tmcl]

@tmcl Would you cherry-pick 06b3dd3 to this PR and update the reason for the pragma?

Done. Also version bump/changelog/since.

[kazu-yamamoto]

@vdukhovni Would you review this PR?

[kazu-yamamoto]

I recalled this issue: haskell-tls/hs-tls#479

[kazu-yamamoto]

Merged and a new version has been released.
Thank you for your contribution!