accesslog: fallback to meshconfig when built-in provider's format is unset

[zirain]

Please provide a description of this PR:

this's a follow up of #39521 (comment)

cc @douglas-reid @ramaraochavali

[GregHanson]

Release note looks good

[zirain]

/retest

[istio-testing]

In response to a cherrypick label: #40851 failed to apply on top of branch "release-1.15":

Applying: accesslog: fallback to meshconfig when built-in provider's format is unset
Using index info to reconstruct a base tree...
M	pilot/pkg/model/telemetry_logging.go
M	pilot/pkg/model/telemetry_logging_test.go
Falling back to patching base and 3-way merge...
Auto-merging pilot/pkg/model/telemetry_logging_test.go
Auto-merging pilot/pkg/model/telemetry_logging.go
CONFLICT (content): Merge conflict in pilot/pkg/model/telemetry_logging.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 accesslog: fallback to meshconfig when built-in provider's format is unset
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

[istio-testing]

In response to a cherrypick label: new issue created for failed cherrypick: #41085