Changes to sdf::ParamPrivate should not break ABI

## Desired behavior
Per the Pimpl idiom, one would expect changes to the `sdf::ParamPrivate` class to not break ABI, but
since the class is in a public header (`Param.h`) and since it's members are used in function templates of `sdf::Param`, any changes made to `sdf::ParamPrivate` could break ABI.

This is similar to https://github.com/ignitionrobotics/sdformat/issues/607

## Alternatives considered
n/a

## Implementation suggestion
Provide accessor functions in `sdf::Param` so that the function templates do not need direct access to `dataPtr`. Then we can move `sdf::ParamPrivate` to `Param.cc`. This would be a breaking change, so it will have to target the `main` branch.

Start by reading Param.h and tracing how the function templates of sdf::Param access dataPtr in sdf::ParamPrivate. Use the proposed Param.cc move and accessor functions as the scope; the work is done when ParamPrivate is no longer exposed through the public header without breaking the intended ABI.

Written by the indexing model from the issue text.