## Why this should be merged
The `temporary.WithTempRegisteredExtras()` global lock introduced in
#234 wasn't fit for purpose when used in `coreth` as it required central
coordination of registration, types, and usage of the payload accessor.
## How this works
Instead of a central registration point, the new
`libevm.WithTemporaryExtrasLock()` function takes out a global lock and
provides the caller with a handle that proves the lock is held. All of
the override functions, e.g. `params.WithTempRegisteredExtras()` now
require a current lock, which will be propagated by the respective
`coreth` functions.
See https://github.com/ava-labs/coreth/pull/1328 for intended usage in
`coreth` and `subnet-evm`. A consumer of both of these can then safely
do the following:
```go
import (
"github.com/ava-labs/libevm/libevm"
coreth "github.com/ava-labs/coreth/plugin/evm"
subnet "github.com/ava-labs/subnet-evm/plugin/evm"
)
// asCChain calls `fn` while emulating `coreth`. It is safe for concurrent usage with [asSubnetEVM].
func asCChain(fn func() error) error {
return libevm.WithTemporaryExtrasLock(func(l libevm.ExtrasLock) error {
return coreth.WithTempRegisteredLibEVMExtras(l, fn)
})
}
// asSubnetEVM calls `fn` while emulating `subnet-evm`. It is safe for concurrent usage with [asCChain].
func asSubnetEVM(fn func() error) error {
return libevm.WithTemporaryExtrasLock(func(l libevm.ExtrasLock) error {
return subnet.WithTempRegisteredLibEVMExtras(l, fn)
})
}
```
## How this was tested
Unit test of the new function plus existing integration tests of all
modified code.
