[release-1.20] add fallback support for PrivateKeyProvider

[istio-testing]

Manual cherrypick required.

#3045 failed to apply on top of branch "release-1.20":

Applying: add fallback support for privatekeyprovider
Using index info to reconstruct a base tree...
M	annotation/annotations.pb.html
M	label/labels.pb.html
M	mesh/v1alpha1/istio.mesh.v1alpha1.pb.html
M	mesh/v1alpha1/proxy.pb.go
M	mesh/v1alpha1/proxy.proto
M	proto.lock
Falling back to patching base and 3-way merge...
Auto-merging proto.lock
Auto-merging mesh/v1alpha1/proxy.proto
Auto-merging mesh/v1alpha1/proxy.pb.go
Auto-merging mesh/v1alpha1/istio.mesh.v1alpha1.pb.html
Auto-merging annotation/annotations.pb.html
CONFLICT (content): Merge conflict in annotation/annotations.pb.html
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 add fallback support for privatekeyprovider
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".

[zirain]

duplicate with #3054