[Backport 6.2] http: Add missing thread_local specifier for static

[scylladbbot]

Fixes #24447

Patch adding this somehow managed to leave out the thread_local specifier. While gnutls cert object can be shared across shards just fine, the actual shared_ptr here cannot, thus we could cause memory errors.

• (cherry picked from commit 4a98c258f6d2df6067eb0d965c11ba7f3342eb1d)

Parent PR: #24514

[scylladbbot]

@elcallio - This PR was marked as draft because it has conflicts Please resolve them and mark this PR as ready for review

[denesb]

62. is EOL, closing.