Use Haskell variables for passing values.

[MarcFontaine]

Currently the cardano-testnet uses filenames (Strings) to reference objects like keys, certs etc.
Two locations in the code refer to the same object by using the same filename.
This means that the data flow is implicit and hard to follow.
Old scheme:

do
cli_call_1 "filenameX" "filenameY"
cli_call_2 "filenameX" "filenameZ"
cli_call_3 "filenameA" "filenameB"

This PR changes the design as follows:

• File names are stored in an opaque File datatype (which wraps the file path).
• A wrapper is used for CLI calls. When a CLI call creates a file, the wrapper returns the corresponding File resource.
• While CLI calls have type IO () the wrapper has type `IO (File,..,File).
• CLI calls don't construct Filenames (i.e. as magic constants) , but instead use the File resource that was created earlier.

The new scheme is

do
(file1, file2) <- cli_call1 arg1 arg2     -- cli_call1 creates file resources file1 and file2
(fileX, fileY) <- cli_call2 file1 file2    -- cli_call2 uses file resourcs file1 and file2

Full transformation to the new scheme for filenames is still WIP.

[MarcFontaine]

This is a stripped down version of #4845.
This PR introduces CLI wrapper functions and changes three CLI calls to use the wrappers.
Transitioning all CLI calls in cardano-testnet to wrapper functions needs to be done step by step
and combined with other changes. (To be done in followup PRs.)

[Jimbo4350]

A nice improvement in terms of size of the PR. I left a few comments that need to be address. In future, do not include function definitions that are not in used in the PR.