Use existing curl cmake target if it already exists

### Is your feature request related to a problem?

The only options for managing the curl dependency is either finding it on the system or via `FetchContent`. It would be useful if the project checked if the `libcurl` or `CURL::libcurl` target already existed instead of always managing the dependency itself. A project may have reason to find and use curl outside of its use with CPR or select a specific version of it.

### Possible Solution

Add an initial check [here](https://github.com/libcpr/cpr/blob/a0ff651296a620ed5fabf7d12cf4905fefc7ce86/CMakeLists.txt#L192) for existing curl targets, to avoid handling the dependency at all if one already exists.

```cmake
if(TARGET libcurl OR TARGET CURL::libcurl)
# nothing needs to be done
elseif(CPR_USE_SYSTEM_CURL)
...
```

### Alternatives

If default behaviour needs to remain the same, then this could be an explicit option instead:
```cmake
if(CPR_USE_EXISTING_CURL_TARGET and NOT (TARGET libcurl OR TARGET CURL::libcurl))
    message(FATAL_ERROR "curl target not found...")
elseif(CPR_USE_SYSTEM_CURL)
...
```

### Additional Context

_No response_

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